deps: resolve :native against DEB_BUILD_ARCH in cross builds

CheckOpts had no build-arch concept: the build-side facts and :native
qualifiers resolved against the host arch, so in a cross build
(-a armhf on amd64) 'Build-Depends: foo:native' looked for an armhf
package where dpkg-checkbuilddeps looks for an amd64 one. CheckOpts
gains build_arch (DEB_BUILD_ARCH), used for :native and the dpkg status
attribution; bracketed arch restrictions keep evaluating against the
host arch.

src/build/mod.rs

@@ -354,6 +354,10 @@ pub fn run_source_build(
                 .get("DEB_HOST_ARCH")
                 .cloned()
                 .unwrap_or_else(|| crate::debian::arch::native().unwrap_or_default()),
+            build_arch: arch_vars
+                .get("DEB_BUILD_ARCH")
+                .cloned()
+                .unwrap_or_else(|| crate::debian::arch::native().unwrap_or_default()),
             build_profiles: profiles.clone(),
             ..Default::default()
         };
@@ -1599,8 +1603,10 @@ mod differential_tests {
             }
             i += 1;
         }
+        let host_arch = crate::debian::arch::native().unwrap_or_else(|_| "amd64".into());
         let opts = crate::debian::deps::CheckOpts {
-            host_arch: crate::debian::arch::native().unwrap_or_else(|_| "amd64".into()),
+            host_arch: host_arch.clone(),
+            build_arch: host_arch,
             build_profiles: profiles,
             ignore_arch,
             ignore_indep,