In "tests fail on old code", the old code must be the merge base

Otherwise we can run into weird inconsistencies with the partial
checkout, when the base reference of the PR is sufficiently older than
main. We already changed the diff to compare against merge base, but
forgot to change the checkout itself.

Author: akuzm

.github/workflows/tests-fail-on-old-code.yaml

@@ -127,7 +127,9 @@ jobs:
     - name: Revert code changes
       if: steps.check.outputs.should_run == 'true'
       run: |
-        git checkout base -- $CODE_PATHS
+        set -eu
+
+        git checkout $(git merge-base base @) -- $CODE_PATHS
         git status
 
     - name: Build TimescaleDB