.github/workflows: add script to check that packages can still be

installed.

This will allow us to confirm the repository's staging state in CI.

.github/workflows/build.yaml

@@ -87,3 +87,11 @@ jobs:
           cd /
           "$here/common/travis/show_files.sh" "$BOOTSTRAP" "$ARCH"
           )
+
+      - name: Verify repository state
+        run: |
+          (
+          here="$(pwd)"
+          cd /
+          "$here/common/travis/check-install.sh" "$BOOTSTRAP" "$ARCH"
+          )