Provide ref fallback value in the check_commits CI

Previously, we introduced a chance dynamically determining the base
branch for the `check_commits` CI job. Unfortunately it used the
`base_ref` variable, which is only set for `pull_request`s, not for
`push`es. Here, we therefore also provide a `ref_name` fallback value,
which should also work for our post-merge 'push' CI.

Author: tnull

.github/workflows/build.yml

@@ -198,9 +198,9 @@ jobs:
           git fetch upstream
           export GIT_COMMITTER_EMAIL="[email protected]"
           export GIT_COMMITTER_NAME="RL CI"
-          git rebase upstream/${{ github.base_ref }}
+          git rebase upstream/${{ github.base_ref || github.ref_name }}
       - name: For each commit, run cargo check (including in fuzz)
-        run: ci/check-each-commit.sh upstream/${{ github.base_ref }}
+        run: ci/check-each-commit.sh upstream/${{ github.base_ref || github.ref_name }}
 
   check_release:
     runs-on: ubuntu-latest