Hide error outputs from checking for git root (#157851)

## Summary
Addresses #157341 

The error comes because STDERR by default is piped to the parent
process's stderr.
- Quench error output of git rev-parse

### Checklist

Co-authored-by: Kibana Machine <42973632+kibanamachine@users.noreply.github.com>

packages/kbn-repo-packages/modern/get_git_repo_root.js

@@ -26,6 +26,7 @@ function getGitRepoRootSync(repoRoot) {
       cwd: repoRoot,
       encoding: 'utf8',
       maxBuffer: Infinity,
+      stdio: 'pipe',
     });
 
     const firstLine = stdout.split('\n')[0];