coveralls: Add `COVERALLS_GIT_BRANCH` (#29458)

Without this, coveralls didn't receive the branch info thus leading to failures when we switched branches.

.github/workflows/test.yml

@@ -52,3 +52,4 @@ jobs:
         if: matrix.node == 10 && github.repository == 'twbs/bootstrap' && github.event_name == 'push'
         env:
           COVERALLS_REPO_TOKEN: "${{ secrets.COVERALLS_REPO_TOKEN }}"
+          COVERALLS_GIT_BRANCH: "${{ github.ref }}"