feat: Add cache-extra-identifier input for matrix job cache separation

[GuillaumeLagrange]

Fixes #32

GITHUB_JOB is unfortunately not enough to discriminate between matrix branches, and there is no built-in identifier from github actions available in the environment variable to do so as far as I know.

This PR adds an optional input to allow users to customize the resulting cache key.

I've tested on my end, let me know if you have a particular procedure that needs to be performed, or if you have a better idea to solve the cache overriding in matrixes