deb: scope the arch-indep build-dep pass to the host arch in cross builds

Without --host-architecture, the second build-dep pass re-resolves the
whole Build-Depends field for the native architecture: apt swaps
host-arch -dev packages for native ones (e.g. libcurl4-gnutls-dev,
whose arch-differing curl-config makes dpkg refuse the co-install) and
breaks the cross build environment.

Per dpkg-checkbuilddeps, both Build-Depends and Build-Depends-Indep
resolve for the host architecture in cross mode, so pass
--host-architecture to the second pass as well. Skip the pass entirely
when the source declares no Build-Depends-Indep.

Add an end-to-end regression test building a package that declares
libdb-dev in both fields and links a host-arch binary against it: the
test only passes if the arch-indep pass did not swap the arm64 -dev
packages for native ones.

src/deb/local.rs

@@ -276,26 +276,39 @@ pub async fn build(
         return Err("Could not install build-dependencies for the build".into());
     }
 
-    // Install arch-independant build dependencies
-    log::debug!("Installing arch-independant build dependencies...");
-    let status = cap(
-        ctx.command("apt-get")
-            .current_dir(package_dir_str)
+    // Install arch-independant build dependencies, only if the source declares
+    // any: without --arch-only this pass resolves the whole Build-Depends field
+    // too, which is redundant after the first pass and breaks cross builds.
+    let has_indep_deps = match ctx.read_file(&package_dir.join("debian/control")) {
+        Ok(control) => control
+            .lines()
+            .any(|l| l.to_ascii_lowercase().starts_with("build-depends-indep:")),
+        Err(e) => {
+            log::debug!("cannot read debian/control for indep build-deps: {}", e);
+            true
+        }
+    };
+    if has_indep_deps {
+        log::debug!("Installing arch-independant build dependencies...");
+        let mut cmd = ctx.command("apt-get");
+        cmd.current_dir(package_dir_str)
             .envs(env.clone())
             .arg("-y")
-            .arg("build-dep")
-            .arg("./"),
-        &sink,
-    )
-    .status()?;
+            .arg("build-dep");
+        if cross {
+            cmd.arg(format!("--host-architecture={arch}"));
+        }
+        cmd.arg("./");
+        let status = cap(&mut cmd, &sink).status()?;
 
-    // If build-dep fails, we try to explain the failure using dose-debcheck
-    if !status.success() {
-        if let Some(u) = &ui {
-            u.suspend();
+        // If build-dep fails, we try to explain the failure using dose-debcheck
+        if !status.success() {
+            if let Some(u) = &ui {
+                u.suspend();
+            }
+            dose3_explain_dependencies(package, version, arch, build_root, cross, ctx.clone())?;
+            return Err("Could not install build-dependencies for the build".into());
         }
-        dose3_explain_dependencies(package, version, arch, build_root, cross, ctx.clone())?;
-        return Err("Could not install build-dependencies for the build".into());
     }
 
     // Run the build step