build.yml: Stop Codecov from failing on forks

Symptom was (with line breaks added):
> Codecov: Failed to properly create commit: The process
> '/home/runner/work/_actions/codecov/codecov-action/v4/dist/codecov'
> failed with exit code 1

.github/workflows/build.yml

@@ -24,6 +24,7 @@ jobs:
  pip install coverage
  - run: python tests/test.py --coverage
  - uses: codecov/codecov-action@v4
+ if: github.repository == 'jrfonseca/gprof2dot'
  with:
  token: ${{ secrets.CODECOV_TOKEN }}