chore: report disk usage in test run

Either the runner code doesn't clean up after itself, we're not
cleaning up after ourselves, or we're just not giving the runner
enough disk space. As a result, we're seeing out of space errors.

This addition will help diagnose the situation.

.github/workflows/tests.yaml

@@ -52,6 +52,11 @@ jobs:
       issues: write
       pull-requests: write
     steps:
+      # This helps understand how much disk space the cache and custom runner
+      # is actually using.
+      # Related to https://github.com/actions/runner/issues/2708
+      - name: Report disk usage
+        run: df -h
       - name: Setup Go
         uses: actions/setup-go@0a12ed9d6a96ab950c8f026ed9f722fe0da7ef32 # v5.0.2
         with: