github: fix base branch on push action

Author: ellemouton

.github/workflows/main.yml

@@ -17,6 +17,12 @@ env:
   # /dev.Dockerfile
   GO_VERSION: 1.18
 
+  # If the action was triggered by a pull request, then the target branch of
+  # the pull request is used as the base branch. However, if the action is a
+  # push, then base_ref will be empty and the branch being pushed to should
+  # instead be used as the base branch.
+  BASE_BRANCH: ${{ github.base_ref || github.ref_name }}
+
 jobs:
   ########################
   # frontend build checks
@@ -219,16 +225,16 @@ jobs:
         with:
           go-version: '${{ env.GO_VERSION }}'
 
-      - name: fetch and rebase on ${{ github.base_ref }}
+      - name: fetch and rebase on ${{ env.BASE_BRANCH }}
         run: |
           git remote add upstream https://github.com/${{ github.repository }}
           git fetch upstream
           export GIT_COMMITTER_EMAIL="[email protected]"
           export GIT_COMMITTER_NAME="LiT CI"
-          git rebase upstream/${{ github.base_ref }}
+          git rebase upstream/${{ env.BASE_BRANCH }}
 
       - name: check commits
-        run: scripts/check-each-commit.sh upstream/${{ github.base_ref }}
+        run: scripts/check-each-commit.sh upstream/${{ env.BASE_BRANCH }}
 
   ########################
   # lint code