Fix Coveralls Action

After we switched to GitHub Actions CI, the branch isn't passed on coveralls.

One way I've been experimenting is using COVERALLS_GIT_BRANCH: "${{ github.ref }}" but this results in refs/head/branch instead of branch, so it needs further manipulation.

There's an official action for coveralls, but it seems it leaves comments even when the setting on coveralls is disabled.

I'll try to find a solution soon.