php-testbench: Github Actions/Workflow woes

[ppelgrims]

Hello!

In order to be compliant with CZ/Wieni policy regarding the use of external actions, I tried to convert a couple we use on the php-testbench project. I was consulting Spoit for a solution regarding php-cs-fixer, alas my attempts are not quite working yet: #24

I'm aware of the phpstan errors related to generics in phpdocs, I'm not sure how easily they can be fixed, but that's out of scope of this issue anyway.

Thank you in advance!

[spoit]

Did some changes, seem to work now: #24

[ppelgrims]

@spoit 🙏 merci!