tests: Add a check for valid snapshot urls

This pulls the list of snapshots from the rpmrepo API, greps the
codebase for all uses of rpmrepo.osbuild.org that look like a snapshot
name, and then checks to make sure they are still valid.

This is the same script as osbuild-composer, except that it also skips
checking ./test/data/stages/ which has rpm urls that look like snapshot
urls.

.github/workflows/check.yml

@@ -44,3 +44,14 @@ jobs:
       with:
         severity: warning
         token: ${{ secrets.GITHUB_TOKEN }}
+
+  snapshots:
+    name: "🔍 Check for valid snapshot urls"
+    runs-on: ubuntu-20.04
+    steps:
+      - name: Check out code into the Go module directory
+        uses: actions/checkout@v3
+        with:
+          ref: ${{ github.event.pull_request.head.sha }}
+      - name: Check for valid snapshot urls
+        run: ./tools/check-snapshots --errors-only .