gh: add code coverage

Author: charlievieth

.github/workflows/linux.yml

@@ -20,8 +20,10 @@ jobs:
         with:
           go-version: ${{ matrix.go }}
       - name: Test
-        run: go test ./...
-      - name: Test Race
-        run: go test -race ./...
+        run: make test
       - name: Test Builds
         run: make test_build --jobs=4
+      - name: Upload coverage reports to Codecov
+        uses: codecov/codecov-action@v5
+        with:
+          token: ${{ secrets.CODECOV_TOKEN }}