external CI checks for PR-specific branch

this allows us to validate that external proofs are ready to merge fixes
before we merge breaking changes, without requiring config fiddling.

Author: fdupress

.github/workflows/ci.yml

@@ -123,10 +123,21 @@ jobs:
     - uses: actions/checkout@v4
       with:
         path: easycrypt
+    - name: Extract target branch name
+      run: echo "branch=refs/heads/merge-${GITHUB_HEAD_REF:-${GITHUB_REF#refs/heads/}}" >> $GITHUB_OUTPUT
+      id: extract_branch
+    - name: Find remote branch
+      id: branch_name
+      run: |
+        git ls-remote --exit-code --heads ${{ matrix.target.repository }} ${{ steps.extract_branch.outputs.branch }} || exists=$?
+        if [ "$exists" = "2" ];
+        then echo "REPO_BRANCH=${{ matrix.target.branch }}" >> $GITHUB_OUTPUT;
+        else echo "REPO_BRANCH=${{ steps.extract_branch.outputs.branch }}" >> $GITHUB_OUTPUT;
+        fi
     - name: Checkout External Project
       run: |
         git clone --recurse-submodules \
-          -b ${{ matrix.target.branch }} \
+          -b ${{ steps.branch_name.outputs.REPO_BRANCH }} \
           ${{ matrix.target.repository }} \
           project/${{ matrix.target.name }}
     - name: Install EasyCrypt dependencies