Point to user code with idempotent plugin warning

[jrbourbeau]

I was just trying to find where a plugin that was using the old-style idempotent was and found this helpful

[github-actions]

Unit Test Results

See test report for an extended history of previous test failures. This is useful for diagnosing flaky tests.

    25 files  ±0      25 suites  ±0   10h 9m 17s ⏱️ -42s
 4 125 tests ±0   4 013 ✅ +6    111 💤 ±0  1 ❌  - 6 
47 651 runs  ±0  45 545 ✅ +6  2 105 💤 ±0  1 ❌  - 6 

For more details on these failures, see this check.

Results for commit 4b9eaa8. ± Comparison against base commit d728052.

[hendrikmakait]

Thanks, @jrbourbeau!