fix: update CodeCov to v4.

CodeCov upload has been consistently failing when trying to upload
coverage for a PR from a fork. This change updates CodeCov to v4 and
sets the `CODECOV_TOKEN` env var in an attempt to fix this issue.

See codecov/codecov-action#598

.github/workflows/test.yml

@@ -23,8 +23,9 @@ jobs:
  - name: Test
  run: go test -v -race -coverprofile=coverage.txt -covermode=atomic ./...
  - name: Report coverage
- uses: codecov/codecov-action@v3
+ uses: codecov/codecov-action@v4
  with:
- token: ${{ secrets.CODECOV_TOKEN }}
  file: coverage.txt
  fail_ci_if_error: true
+ env:
+ CODECOV_TOKEN: ${{ secrets.CODECOV_TOKEN }}