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' \