Update common github actions (#415)

Signed-off-by: Knative Automation <automation@knative.team>

.github/workflows/knative-releasability.yaml

@@ -50,6 +50,9 @@ jobs:
           if [[ "${{ github.event.inputs.slackChannel }}" != "" ]]; then
             echo "SLACK_CHANNEL=${{ github.event.inputs.slackChannel }}" >> $GITHUB_ENV
           fi
+          if [[ "${{ secrets.SLACK_WEBHOOK }}" != "" ]]; then
+            echo "SLACK_WEBHOOK=exists" >> $GITHUB_ENV
+          fi
 
       - name: Set up Go 1.15.x
         uses: actions/setup-go@v2
@@ -142,7 +145,8 @@ jobs:
           echo 'SLACK_TITLE=Releasability for ${{ github.repository }} @ ${{ env.RELEASE }} is NO-GO!' >> $GITHUB_ENV
 
       - name: Post status to Slack
-        if: ${{ secrets.SLACK_WEBHOOK != '' && steps.exists.outputs.release-branch == 'false' }}
+        # Note: using env.SLACK_WEBHOOK here because secrets are not allowed in the if block.
+        if: env.SLACK_WEBHOOK != '' && steps.exists.outputs.release-branch == 'false'
         uses: rtCamp/action-slack-notify@v2.1.0
         env:
           SLACK_ICON: http://github.com/knative.png?size=48