Build Linux x64 against an isolated sysroot.

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

@@ -43,11 +43,7 @@ case "$target" in
             "$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
-        # while resolving foreign-architecture development packages.
-        sudo env DEBIAN_FRONTEND=noninteractive apt-get remove -y mercurial python3-importlib-metadata python3-jwt
-        install_packages python3 python3-typing-extensions
+        install_packages build-essential cmake gcc-x86-64-linux-gnu g++-x86-64-linux-gnu pkg-config
 
         sudo dpkg --add-architecture amd64
         {
@@ -67,15 +63,22 @@ case "$target" in
                 'Signed-By: /usr/share/keyrings/ubuntu-archive-keyring.gpg'
         } | sudo tee /etc/apt/sources.list.d/ruds-amd64.sources >/dev/null
         sudo apt-get update
-        install_packages \
-            build-essential \
-            cmake \
-            gcc-x86-64-linux-gnu \
-            g++-x86-64-linux-gnu \
+
+        # Foreign development packages contain architecture-specific helper
+        # programs whose maintainer scripts cannot run on the arm64 host.
+        # Download them without installing and extract a link-only sysroot.
+        sysroot=/opt/ruds-x64-sysroot
+        archives=/tmp/ruds-x64-archives
+        sudo mkdir -p "$archives/partial" "$sysroot"
+        sudo env DEBIAN_FRONTEND=noninteractive apt-get install -y --no-install-recommends \
+            --download-only \
+            -o Dir::Cache::archives="$archives" \
             libgtk-3-dev:amd64 \
             libwebkit2gtk-4.1-dev:amd64 \
-            libxdo-dev:amd64 \
-            pkg-config
+            libxdo-dev:amd64
+        for package in "$archives"/*.deb; do
+            sudo dpkg-deb --extract "$package" "$sysroot"
+        done
         ;;
     *)
         echo "unsupported Linux build target: $target" >&2