validate-pr: missing `skipped` output for admin/maintainer authors

[jpnurmi]

When the PR author has admin or maintain access, the action logs a skip message and returns early but does not set core.setOutput('skipped', 'true'). The allowed-bots path does set this output.

This means the "Convert PR to draft" step still runs for maintainers, because steps.validate.outputs.skipped is unset:

if: >-
  steps.validate.outputs.was-closed != 'true'
  && steps.validate.outputs.skipped != 'true'
  && github.event.pull_request.draft == false

Failed run: https://github.com/getsentry/sentry-native/actions/runs/23681625659/job/68994397549?pr=1606

Relevant code (validate-pr/scripts/validate-pr.js):

Allowed bots (correct):

if (ALLOWED_BOTS.includes(prAuthor)) {
  core.info(`PR author ${prAuthor} is an allowed bot. Skipping.`);
  core.setOutput('skipped', 'true');
  return;
}

Maintainer check (missing output):

if (authorIsMaintainer) {
  core.info(`PR author ${prAuthor} has admin/maintain access. Skipping.`);
  return; // <-- no setOutput('skipped', 'true')
}

Expected behavior: Maintainers should be fully skipped, including the draft conversion step.

Fix: Add core.setOutput('skipped', 'true'); before the return in the maintainer check.