Fix git diff path in CI workflow

Working directory is src/current, so the path to snapshot_relative.py
should be relative to that, not the repo root. Also add -- to
disambiguate the path from a git revision.

Author: ebembi-crdb

.github/workflows/test-archive-scripts.yml

@@ -38,4 +38,6 @@ jobs:
 
       - name: Verify snapshot_relative.py is unchanged
         # Confirms the scripts never modify the checked-in source file.
-        run: git diff --exit-code src/current/snapshot_relative.py
+        # Use -- to disambiguate path from a revision; path is relative to
+        # working-directory (src/current).
+        run: git diff --exit-code -- snapshot_relative.py