CI: stop running coveralls in forks

[XhmikosR]

No description provided.

[XhmikosR]

Not sure if this will work for PRs... It probably won't. My plan is to stop this from running when we are not on our repo or a PR against our repo.

[julien-deramond]

Maybe both are working but I've seen and used another approach to do that:

Suggested change

	if: github.repository == 'twbs/bootstrap'
	if: github.repository_owner == 'twbs'

If I remember well, the job starts but is canceled because of the condition.

[XhmikosR]

I think we could probably just use this too:

if: ${{ !github.event.repository.fork }}

[louismaximepiton]

Just tried it on a forked Bootstrap and it seems to work quite well. Didn't tried inside a PR against our repo.