tools: make update-undici script executable

PR-URL: #42406
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: Rich Trott <[email protected]>
Reviewed-By: Tobias Nießen <[email protected]>
Reviewed-By: Darshan Sen <[email protected]>
Reviewed-By: Luigi Pinca <[email protected]>