fix: appease linter on darwin (#11154)

Fixing up some linting errors that show up on Darwin, but not in CI.

agent/usershell/usershell_darwin.go

@@ -13,6 +13,10 @@ import (
 func Get(username string) (string, error) {
 	// This command will output "UserShell: /bin/zsh" if successful, we
 	// can ignore the error since we have fallback behavior.
+	if !filepath.IsLocal(username) {
+		return "", xerrors.Errorf("username is nonlocal path: %s", username)
+	}
+	//nolint: gosec // input checked above
 	out, _ := exec.Command("dscl", ".", "-read", filepath.Join("/Users", username), "UserShell").Output()
 	s, ok := strings.CutPrefix(string(out), "UserShell: ")
 	if ok {