deb: export the dpkg architecture environment for the rules

The rules run outside dpkg-buildpackage, which exports none of the
dpkg architecture variables anyway - only sbuild does. Packages using
$(DEB_HOST_MULTIARCH) or $(DEB_BUILD_GNU_TYPE) without including
/usr/share/dpkg/architecture.mk expanded them empty: micro's rules
cd'd into 'obj-' and file configured --libdir=/usr/lib, putting its
libraries where dh_install's multiarch glob could not find them.

Run 'dpkg-architecture -s' in the build context and export the whole
DEB_* set, sbuild-style, for native builds; cross builds already carry
the target's set from cross::setup_environment, whose output parser
this reuses.

src/deb/cross.rs

@@ -7,7 +7,9 @@ use std::sync::Arc;
 /// Parse 'dpkg-architecture' output (KEY=value lines) into a set of
 /// environment variables. Unexpected lines (e.g. warnings on stderr leaking
 /// into stdout) are skipped instead of causing a failure.
-fn parse_dpkg_architecture_output(output: &str, env: &mut HashMap<String, String>) {
+/// Parse the `VAR=value; ...; export ...` output of `dpkg-architecture -s`
+/// into `env`.
+pub fn parse_dpkg_architecture_output(output: &str, env: &mut HashMap<String, String>) {
     let env_var_regex = regex::Regex::new(r"(?<key>.*)=(?<value>.*)").unwrap();
     for l in output.lines() {
         let Some(capture) = env_var_regex.captures(l) else {

src/deb/local.rs

@@ -152,6 +152,25 @@ pub async fn build(
         log::debug!("Setting up environment for local cross build...");
         cross::setup_environment(&mut env, arch, ctx.clone())?;
         cross::ensure_repositories(arch, series, pocket, mirror, ctx.clone())?;
+    } else {
+        // sbuild-style environment for debian/rules: the rules run outside
+        // dpkg-buildpackage, which exports none of the dpkg architecture
+        // variables — packages using $(DEB_HOST_MULTIARCH) and friends
+        // without including /usr/share/dpkg/architecture.mk would expand
+        // them empty. Cross builds already carry the target's set from
+        // cross::setup_environment.
+        let output = ctx
+            .command("dpkg-architecture")
+            .output()
+            .map_err(|e| format!("cannot run dpkg-architecture: {e}. Is 'dpkg-dev' installed?"))?;
+        if !output.status.success() {
+            return Err(
+                format!("'dpkg-architecture' failed with status: {}", output.status).into(),
+            );
+        }
+        let dpkg_architecture = String::from_utf8(output.stdout)
+            .map_err(|e| format!("Invalid UTF-8 in dpkg-architecture output: {e}"))?;
+        cross::parse_dpkg_architecture_output(&dpkg_architecture, &mut env);
     }
 
     let mut sources = apt::sources::load(Some(ctx.clone()))?;