github: Add a manual job for testing installing with a system clang

Author: mstorsjo

.github/workflows/test-system-clang.yml

@@ -0,0 +1,15 @@
+name: Test installing with a system clang
+on:
+  workflow_dispatch:
+
+jobs:
+  docker-build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - name: Build Docker images
+        uses: docker/build-push-action@v5
+        with:
+          context: .
+          push: false
+          file: ./Dockerfile.system-clang