Have check_commit run against base branch

Author: tnull

.github/workflows/build.yml

@@ -215,9 +215,9 @@ jobs:
           git fetch upstream
           export GIT_COMMITTER_EMAIL="[email protected]"
           export GIT_COMMITTER_NAME="RL CI"
-          git rebase upstream/main
+          git rebase upstream/${{ github.base_ref }}
       - name: For each commit, run cargo check (including in fuzz)
-        run: ci/check-each-commit.sh upstream/main
+        run: ci/check-each-commit.sh upstream/${{ github.base_ref }}
 
   check_release:
     runs-on: ubuntu-latest