Refresh native apt indexes before cross-build setup.

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

@@ -41,6 +41,7 @@ case "$target" in
         sudo sed -i \
             '/^Architectures:/d; /^Types: deb$/a Architectures: arm64' \
             "$native_sources"
+        sudo apt-get update
 
         # The full runner image includes these Architecture: all Python tools.
         # They are unrelated to the build, but make apt replace native Python

crates/bds-ui/tests/packaging_assets.rs

@@ -214,6 +214,13 @@ fn tagged_releases_are_built_and_packaged_with_rust_tools() {
     let native_python = linux_dependencies
         .find("install_packages python3 python3-typing-extensions")
         .expect("native Python dependency setup");
+    let native_source_filter = linux_dependencies
+        .find("'/^Architectures:/d; /^Types: deb$/a Architectures: arm64'")
+        .expect("native apt architecture filter");
+    assert!(
+        linux_dependencies[native_source_filter..native_python].contains("sudo apt-get update"),
+        "native apt indexes must be refreshed after constraining the source architecture"
+    );
     assert!(
         native_python < multiarch,
         "native Python dependencies must be fixed before enabling multiarch"