build: check stat's exit status and honor DPKG_ORIGINS_DIR

hashes_in_context never checked stat's exit status and parsed its size
with unwrap_or(0), silently recording zero-size artifacts in the
generated .changes/.buildinfo; stat failures and unparsable sizes are
now errors naming the file. current_vendor hardcoded
/etc/dpkg/origins/default while dpkg honors DPKG_ORIGINS_DIR (already
in the file's own ENV_ALLOWED list); the origins default is now
resolved against it with the usual fallback.

src/build/binary.rs

@@ -332,21 +332,34 @@ fn hashes_in_context(
         .map(|n| (n.clone(), ArtifactHashes::default()))
         .collect();
 
-    // Sizes.
+    // Sizes. A failed `stat` must fail the metadata generation: an unchecked
+    // exit status would leave the default size 0 in the produced
+    // `.changes`/`.buildinfo` checksum entries.
     let output = ctx
         .command("stat")
         .current_dir(dir)
         .arg("-c")
         .arg("%s %n")
         .args(names)
-        .output()?;
+        .output()
+        .map_err(|e| format!("failed to run 'stat' inside the build context: {e}"))?;
+    if !output.status.success() {
+        return Err(format!(
+            "'stat' failed inside the build context: {}",
+            String::from_utf8_lossy(&output.stderr).trim()
+        )
+        .into());
+    }
     let stdout = String::from_utf8_lossy(&output.stdout);
     for line in stdout.lines() {
         let Some((size, name)) = line.trim().split_once(' ') else {
             continue;
         };
+        let size = size
+            .parse::<u64>()
+            .map_err(|_| format!("'stat' reported an invalid size '{size}' for '{name}'"))?;
         if let Some(slot) = out.get_mut(name) {
-            slot.size = size.parse().unwrap_or(0);
+            slot.size = size;
         }
     }