ci: skip generate-tool-specs job on fork PRs

GitHub doesn't expose repo secrets to pull_request events from forks, so
${{ secrets.CREWAI_TOOL_SPECS_APP_ID }} resolves to an empty string and
tibdex/github-app-token@v2 errors with "Input required and not supplied:
app_id". The job also tries to push commits to the PR branch, which it
can't do on a fork regardless. Skip it for cross-repo PRs and keep it
for same-repo PRs and manual dispatch.

Co-authored-by: Greyson LaLonde <greyson.r.lalonde@gmail.com>