ci: Use codecov token

This wasn't required for PRs but for the job that runs on the `master`
branch, the job fails without the token (because it's a protected
branch)

.github/workflows/ci.yml

@@ -146,6 +146,7 @@ jobs:
         if: ${{ matrix.polybar_build_type == 'tests' }}
         uses: codecov/codecov-action@v4
         with:
+          token: ${{ secrets.CODECOV_TOKEN }}
           flags: unittests
           # We provide our own coverage files
           disable_search: true