Isolate Linux cross-build dependencies.

.gitea/scripts/install-linux-build-dependencies.sh

@@ -42,6 +42,11 @@ case "$target" in
             '/^Architectures:/d; /^Types: deb$/a Architectures: arm64' \
             "$native_sources"
 
+        # The full runner image includes these Architecture: all Python tools.
+        # They are unrelated to the build, but make apt replace native Python
+        # while resolving foreign-architecture development packages.
+        sudo env DEBIAN_FRONTEND=noninteractive apt-get remove -y mercurial python3-importlib-metadata python3-jwt
+
         sudo dpkg --add-architecture amd64
         {
             printf '%s\n' \

crates/bds-ui/tests/packaging_assets.rs

@@ -169,12 +169,23 @@ fn tagged_releases_are_built_and_packaged_with_rust_tools() {
         "libwebkit2gtk-4.1-dev:amd64",
         "gcc-x86-64-linux-gnu",
         "Architectures: arm64",
+        "remove -y mercurial python3-importlib-metadata python3-jwt",
     ] {
         assert!(
             linux_dependencies.contains(required),
             "Linux dependency setup is missing {required}"
         );
     }
+    let cleanup = linux_dependencies
+        .find("remove -y mercurial python3-importlib-metadata python3-jwt")
+        .expect("runner Python cleanup");
+    let multiarch = linux_dependencies
+        .find("dpkg --add-architecture amd64")
+        .expect("amd64 architecture setup");
+    assert!(
+        cleanup < multiarch,
+        "runner-only Python packages must be removed before enabling multiarch"
+    );
 
     let workspace_manifest = fs::read_to_string(workspace.join("Cargo.toml")).unwrap();
     let core_manifest = fs::read_to_string(workspace.join("crates/bds-core/Cargo.toml")).unwrap();