CI: Improve Infection setup

[Wirone]

• Remove Infection before building PHAR
• Run Infection step only for Pull Requests

[coveralls]

coverage: 96.035%. remained the same
when pulling bfef10a on Wirone:codito/infection-appendix
into a6098f8 on PHP-CS-Fixer:master.

[Wirone]

FYI: for now it's a bit hacky as it still triggers the Infection job, but the actual step is skipped. It would be better if job wasn't included in the matrix at all, but I don't know how to do it properly. It can be improved later if needed.

[keradus]

I think it's fine.
Possible to move it to separate file and have condition for file and not for step, but maybe it's overkill (I'm not asking to follow this path)