Make GitHub annotation work when Infection is run in subdirectory

[kubawerlos]

When Infection is run in Git's repository root (with ./backend/vendor/bin/infection) then logged warnings are fine:

::warning file=backend/src/Foo.php,line=42::Escaped Mutant for Mutator (...)

However, when Infection is run in subdirectory (backend, with command ./vendor/bin/infection) then logged warnings are like this:

::warning file=src/Foo.php,line=42::Escaped Mutant for Mutator (...)

which makes Github annotation not working.

This change fixes that problem.

[maks-rafalko]

Thank you @kubawerlos