Use env variable

Signed-off-by: Christian König <ckoenig@posteo.de>

.github/workflows/stale.yml

@@ -23,7 +23,7 @@ jobs:
         days-before-stale: 30
         days-before-close: 5
         stale-issue-message: 'This issue is stale because it has been open 30 days with no activity. Please comment or update this issue or it will be closed in 5 days.'
-        stale-issue-label: $stale_label
+        stale-issue-label: '${{ env.stale_label }}'
         exempt-issue-labels: 'pinned, Fixed in next release, bug, never-stale, documentation, investigating'
         exempt-all-issue-assignees: true
         operations-per-run: 300
@@ -42,7 +42,7 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v3.4.0
       - name: Remove 'stale' label
-        run: gh issue edit ${{ github.event.issue.number }} --remove-label $stale_label
+        run: gh issue edit ${{ github.event.issue.number }} --remove-label ${{ env.stale_label }}
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}