CI: only generate a contrib tarball if we're on the main branch
There's no need to generate the tarball since it's not going to be used by anyone else. It's only used on master after a merge or in nightly builds.
This should save a few minutes during each contrib MR builds.
The values are
| pipeline type | CI_COMMIT_BRANCH | CI_DEFAULT_BRANCH |
|---|---|---|
| MR pipeline | '' | 'master' |
| merged on master | 'master' | 'master' |
| nightly build | 'master' | 'master' |
Edited by Steve Lhomme