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