CI: Include explicit `failure()` condition in "recent base" check

This is necessary to override the default `success()` status check, and
allow the narrowing condition to function correctly.

.github/workflows/checks.yml

@@ -21,11 +21,11 @@ jobs:
         run: git merge-base --is-ancestor 267ddf8efe36cc799c4c31772a8883ea332ef55b HEAD
 
       - name: Tell PR author if they need to rebase
-        if: steps.tekton.outcome == 'failure'
+        if: failure() && steps.tekton.outcome == 'failure'
         run: echo "::error::Branch needs to be rebased so that Tekton CI can run"
 
       - name: Avoid running Tekton CI if it would provably fail
-        if: steps.tekton.outcome == 'failure'
+        if: failure() && steps.tekton.outcome == 'failure'
         uses: actions/github-script@v6
         with:
           script: |