github: Fix codecov uploads

The old token-less method of uploading coverage metrics to Codecov no
longer works. Instead, one gets a "rate limited" message:

    [2024-06-16T07:05:19.475Z] ['error'] There was an error running the uploader: Error uploading to https://codecov.io: Error: There was an error fetching the storage URL during POST: 429 - {'detail': ErrorDetail(string='Rate limit reached. Please upload with the Codecov repository upload token to resolve issue. Expected time to availability: 1522s.', code='throttled')}
    [2024-06-16T07:05:19.475Z] ['verbose'] The error stack is: Error: Error uploading to https://codecov.io: Error: There was an error fetching the storage URL during POST: 429 - {'detail': ErrorDetail(string='Rate limit reached. Please upload with the Codecov repository upload token to resolve issue. Expected time to availability: 1522s.', code='throttled')}

Upgrading the codecov action version and adding the token re-enables
coverage uploading.

.github/workflows/workflow.yml

@@ -42,8 +42,9 @@ jobs:
             genhtml build/cov_nosys.info -o build/lcov
 
       - name: Upload coverage to Codecov
-        uses: codecov/codecov-action@v2
+        uses: codecov/codecov-action@v4
         with:
+          token: ${{ secrets.codecov_token }}
           flags: unittests
           files: build/cov_nosys.info
           name: codecov-umbrella # optional