fix(ci): only submit coverage from ubuntu (macOS runner rejects the coveralls Homebrew tap)

GitHub's macOS runners now refuse to load the untrusted coverallsapp/coveralls
Homebrew tap, causing the 'Submit coverage results' step to fail on macOS and
fail-fast to cancel the rest of the matrix. Scope that step to ubuntu-latest;
coverage from a single OS is sufficient and this also avoids per-node-version
flag-name collisions across operating systems.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: Jesse Wright

.github/workflows/ci.yml

@@ -47,6 +47,7 @@ jobs:
       - name: Run webpack
         run: yarn run test:webpack
       - name: Submit coverage results
+        if: matrix.os == 'ubuntu-latest'
         uses: coverallsapp/github-action@v2
         with:
           github-token: ${{ secrets.github_token }}