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.
Checklist
-
make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
documentation is changed or added -
commit message follows commit guidelines