doc: update job reference in COLLABORATOR_GUIDE.md

https://ci.nodejs.org/job/node-test-commit-custom-suites/ is disabled
in Jenkins with a note to instead use
https://ci.nodejs.org/job/node-test-commit-custom-suites-freestyle/.
Update the guide to remove the layer of indirection.

PR-URL: https://github.com/nodejs/node/pull/31557


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>