workaround for gcov2lcov warning

```
2024/10/06 13:15:37 warn: go/build: go list github.com/sue445/go-gem-wrapper/ruby/: exit status 1
go: inconsistent vendoring in /home/runner/work/go-gem-wrapper/go-gem-wrapper:
	github.com/stretchr/testify@v1.9.0: is explicitly required in go.mod, but not marked as explicit in vendor/modules.txt
	github.com/davecgh/go-spew@v1.1.1: is explicitly required in go.mod, but not marked as explicit in vendor/modules.txt
	github.com/pmezard/go-difflib@v1.0.0: is explicitly required in go.mod, but not marked as explicit in vendor/modules.txt
	gopkg.in/yaml.v3@v3.0.1: is explicitly required in go.mod, but not marked as explicit in vendor/modules.txt
	To ignore the vendor directory, use -mod=readonly or -mod=mod.
	To sync the vendor directory, run:
		go mod vendor
```

https://github.com/sue445/go-gem-wrapper/actions/runs/11202179509/job/31137980923?pr=182

Author: sue445

.github/workflows/build.yml

@@ -61,6 +61,9 @@ jobs:
         run: bundle exec rake
         working-directory: ${{ env.TEST_GEM_DIR }}
 
+      # FIXME: workaround for gcov2lcov warning
+      - run: rm -rf vendor/
+
       - name: Convert coverage to lcov
         uses: jandelgado/gcov2lcov-action@v1
         with: