Add 'no_stale' label for actions/stale (learning-process#410)

Author: aobolensk

.github/workflows/nightly.yml

@@ -22,3 +22,4 @@ jobs:
           days-before-issue-close: -1
           stale-pr-label: 'stale'
           operations-per-run: 100
+          exempt-pr-labels: 'no_stale'