build: record the actual build environment in .buildinfo

The binary build exported DEB_BUILD_OPTIONS='parallel=<context nproc>
nocheck' (or the -j override) but the generated .buildinfo recomputed
the environment from host state: host core count, no nocheck, and
vendor profiles that ignored DEB_BUILD_PROFILES (a cross build recorded
no 'cross' profile). generate_binary_metadata now records the exact env
map that was exported to the build steps, and the recorded profiles
come from the exported DEB_BUILD_PROFILES when set.

Also unifies vendor parsing on one helper (the context-side copy lacked
the Origin: fallback of the source-build path).

src/build/binary.rs

@@ -36,10 +36,13 @@ pub struct BinaryMetadataOptions {
     pub profiles: Vec<String>,
     /// Vendor name (`Build-Origin`).
     pub vendor: String,
-    /// Parallel job count advertised in `DEB_BUILD_OPTIONS`.
-    pub parallel: usize,
-    /// Reproducible-builds epoch exported to the build.
-    pub source_date_epoch: i64,
+    /// Environment variables pkh exported to the build steps (e.g. `LANG`,
+    /// `DEB_BUILD_OPTIONS` with the real parallel count and `nocheck`,
+    /// `SOURCE_DATE_EPOCH`, cross `DEB_*` variables). Recorded — filtered to
+    /// dpkg's allow-list — in the `.buildinfo` `Environment` field, taking
+    /// precedence over whatever the host process inherited, so the metadata
+    /// describes the environment the build actually ran in.
+    pub exported_env: BTreeMap<String, String>,
     /// Build architecture (the machine inside the build context).
     pub build_arch: String,
     /// Host architecture (the packages' target); equals the build
@@ -210,8 +213,10 @@ pub fn generate_binary_metadata(
     // ------------------------------------------------------------------
     // .buildinfo generation, then registration in debian/files
     // ------------------------------------------------------------------
-    let pipeline_env = pipeline_environment(opts);
-    let environment = crate::build::env::buildinfo_environment(&pipeline_env);
+    // Record exactly the environment that was exported to the build steps,
+    // overriding any host-inherited value (dpkg-style allowed-variable
+    // filtering, export precedence).
+    let environment = crate::build::env::buildinfo_environment(&opts.exported_env);
 
     // dpkg-genbuildinfo sorts the accumulated architecture values, while
     // dpkg-genchanges keeps encounter order.
@@ -294,24 +299,6 @@ pub fn generate_binary_metadata(
     Ok((buildinfo_path, changes_path))
 }
 
-/// Environment exported to the build steps; recorded (filtered) in the
-/// `.buildinfo` `Environment` field.
-fn pipeline_environment(opts: &BinaryMetadataOptions) -> BTreeMap<String, String> {
-    let mut env = BTreeMap::new();
-    env.insert(
-        "SOURCE_DATE_EPOCH".to_string(),
-        opts.source_date_epoch.to_string(),
-    );
-    env.insert(
-        "DEB_BUILD_OPTIONS".to_string(),
-        format!("parallel={}", opts.parallel),
-    );
-    if !opts.profiles.is_empty() {
-        env.insert("DEB_BUILD_PROFILES".to_string(), opts.profiles.join(","));
-    }
-    env
-}
-
 /// Compute md5/sha1/sha256 digests and sizes for the named files inside the
 /// context directory `dir`, using coreutils.
 fn hashes_in_context(
@@ -463,6 +450,39 @@ fn include_dsc_artifacts(
 mod tests {
     use super::*;
 
+    /// The recorded `.buildinfo` `Environment` must carry the environment
+    /// actually exported to the build steps (`parallel=N nocheck`, `LANG=C`,
+    /// ...), taking precedence over any host-inherited value, instead of
+    /// values recomputed from host state at generation time.
+    #[test]
+    fn environment_records_exported_env_not_host_defaults() {
+        let mut exported_env = BTreeMap::new();
+        exported_env.insert("LANG".to_string(), "C".to_string());
+        exported_env.insert(
+            "DEB_BUILD_OPTIONS".to_string(),
+            "parallel=7 nocheck".to_string(),
+        );
+        let opts = BinaryMetadataOptions {
+            profiles: Vec::new(),
+            vendor: "debian".to_string(),
+            exported_env,
+            build_arch: "amd64".to_string(),
+            host_arch: "amd64".to_string(),
+        };
+
+        let environment = crate::build::env::buildinfo_environment(&opts.exported_env);
+        assert!(
+            environment.contains("DEB_BUILD_OPTIONS=\"parallel=7 nocheck\""),
+            "recorded Environment must carry the exported DEB_BUILD_OPTIONS: {environment}"
+        );
+        assert!(
+            environment.contains("LANG=\"C\""),
+            "recorded Environment must carry the exported LANG: {environment}"
+        );
+        // Not in dpkg's allowed-variable list: never recorded.
+        assert!(!environment.contains("DEBIAN_FRONTEND"), "{environment}");
+    }
+
     /// A minimal previous-version `.dsc` with a 3-column Checksums-Sha1
     /// field, a 4-column Checksums-Sha256 line and a 3-column `Files`.
     /// Regression: the old code filled `names` from any line with a third