[chore] lint fix in cmd/checkapi (open-telemetry#31723)

**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:** <Issue number if applicable>
- open-telemetry#31240


**Testing:** <Describe what testing was performed and which tests were
added.>

**Documentation:** <Describe the documentation added.>

cmd/checkapi/main.go

@@ -47,10 +47,11 @@ func run(folder string, allowlistFilePath string) error {
  }
  allowlist := strings.Split(string(allowlistData), "\n")
  var errs []error
- err = filepath.Walk(folder, func(path string, info fs.FileInfo, err error) error {
+ err = filepath.Walk(folder, func(path string, info fs.FileInfo, _ error) error {
  if info.Name() == "go.mod" {
  base := filepath.Dir(path)
- relativeBase, err := filepath.Rel(folder, base)
+ var relativeBase string
+ relativeBase, err = filepath.Rel(folder, base)
  if err != nil {
  return err
  }
@@ -67,7 +68,7 @@ func run(folder string, allowlistFilePath string) error {
  return nil
  }
  }
- if err := walkFolder(base, componentType); err != nil {
+ if err = walkFolder(base, componentType); err != nil {
  errs = append(errs, err)
  }
  }