[chore] lint fix in cmd/checkapi

[led0nk]

Description:
linting error with:

INFO [runner] linters took 1.336744316s with stages: goanalysis_metalinter: 1.334941582s 
opentelemetry-collector-contrib/cmd/checkapi/main.go:50:66: unused-parameter: parameter 'err' seems to be unused, consider removing or renaming it as _ (revive)
	err = filepath.Walk(folder, func(path string, info fs.FileInfo, err error) error {
	                                                                ^

and:

INFO [runner] linters took 1.806204856s with stages: goanalysis_metalinter: 1.80522593s 
opentelemetry-collector-contrib/cmd/checkapi/main.go:53:18: shadow: declaration of "err" shadows declaration at line 44 (govet)
			relativeBase, err := filepath.Rel(folder, base)
			              ^

Link to tracking Issue:

• [chore] Fix new lint issues with golangci-lint v1.56.1 #31240

Testing:

Documentation:

[linux-foundation-easycla]

• ✅login: led0nk / (a0eeaa8)

The committers listed above are authorized under a signed CLA.

[crobert-1]

Thanks for fixing!