Disable coveralls trigger for forks

[daniyarchambylov]

Circle CI adds environment variable CIRCLE_PR_USERNAME whenever PR is created from forked repo, but doesn't set from main repo's branch.

[coveralls]

Pull Request Test Coverage Report for Build 376

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 82.578%

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Totals
Change from base Build 373:	0.0%
Covered Lines:	247
Relevant Lines:	292

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💛 - Coveralls

[rocky]

And off we go!