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/env.rs

@@ -60,11 +60,16 @@ pub fn arch_env(host_arch: Option<&str>) -> Result<BTreeMap<String, String>, Str
 /// Read the current vendor name from `/etc/dpkg/origins/default`
 /// (its `Vendor:` or `Origin:` field), defaulting to `"debian"`.
 pub fn current_vendor() -> String {
-    read_vendor_from(Path::new("/etc/dpkg/origins/default")).unwrap_or_else(|| "debian".to_string())
+    std::fs::read_to_string(Path::new("/etc/dpkg/origins/default"))
+        .ok()
+        .and_then(|content| vendor_from_origins_content(&content))
+        .unwrap_or_else(|| "debian".to_string())
 }
 
-fn read_vendor_from(path: &Path) -> Option<String> {
-    let content = std::fs::read_to_string(path).ok()?;
+/// Extract the vendor name from the content of a dpkg origins file: its
+/// `Vendor:` field, falling back to `Origin:` when absent. `None` when
+/// neither field carries a non-empty value.
+pub fn vendor_from_origins_content(content: &str) -> Option<String> {
     for line in content.lines() {
         if let Some(value) = line.strip_prefix("Vendor:") {
             let v = value.trim();
@@ -282,6 +287,25 @@ mod tests {
         );
     }
 
+    #[test]
+    fn vendor_from_origins_content_prefers_vendor_then_origin() {
+        assert_eq!(
+            vendor_from_origins_content("Vendor: Ubuntu\nSuite: noble\n"),
+            Some("Ubuntu".to_string())
+        );
+        // Origin fallback when no Vendor field is present.
+        assert_eq!(
+            vendor_from_origins_content("Origin: Debian\nSuite: stable\n"),
+            Some("Debian".to_string())
+        );
+        // Empty Vendor falls through to Origin.
+        assert_eq!(
+            vendor_from_origins_content("Vendor:   \nOrigin: Debian\n"),
+            Some("Debian".to_string())
+        );
+        assert_eq!(vendor_from_origins_content("Suite: stable\n"), None);
+    }
+
     #[test]
     fn environment_escaping() {
         // The function reads the process env; just verify formatting helpers