f Determine BASE_BRANCH programmatically

Author: tnull

.github/workflows/build.yml

@@ -198,9 +198,10 @@ jobs:
           git fetch upstream
           export GIT_COMMITTER_EMAIL="[email protected]"
           export GIT_COMMITTER_NAME="RL CI"
-          git rebase upstream/0.1
+          export BASE_BRANCH="${GITHUB_HEAD_REF:-${GITHUB_REF#refs/heads/}}"
+          git rebase upstream/"$BASE_BRANCH"
       - name: For each commit, run cargo check (including in fuzz)
-        run: ci/check-each-commit.sh upstream/0.1
+        run: ci/check-each-commit.sh upstream/"$BASE_BRANCH"
 
   check_release:
     runs-on: ubuntu-latest