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/mod.rs

@@ -1439,7 +1439,7 @@ Provides: virtual-thing (= 2.0), plain-virtual
         let vendor = env::current_vendor();
         let profiles = env::resolve_build_profiles(&[], &vendor);
         let parallel = env::num_parallel();
-        let build_env_vars: Vec<(String, String)> = [
+        let build_env_vars: BTreeMap<String, String> = [
             ("LANG".to_string(), "C".to_string()),
             (
                 "DEB_BUILD_OPTIONS".to_string(),
@@ -1467,8 +1467,9 @@ Provides: virtual-thing (= 2.0), plain-virtual
         let opts = crate::build::binary::BinaryMetadataOptions {
             profiles,
             vendor,
-            parallel,
-            source_date_epoch: entry.timestamp,
+            // The metadata records exactly the environment exported to the
+            // build steps above.
+            exported_env: build_env_vars,
             build_arch: native_arch.clone(),
             host_arch: native_arch,
         };