Get matrix from inside an action

[sagikazarmark]

Describe the enhancement
I'd like to access the matrix from a custom action.

Additional information
I'm working on an action that uploads a couple log files as artifacts after the job is complete. I'm trying to keep the action as minimal as possible, so it can work without specifying inputs (using defaults).

By default, the artifact name is formatted as JOB_ID-log. Obviously, this name is ambiguous when there is a build matrix. I'd like to use the matrix to generate a unique name, so I can upload each of them to the artifact store. Unfortunately, I can't find an easy way to access the matrix information using toolkit.

I considered the following alternatives:

• Use a unique ID: I'd like to match the artifact name to the actual build, so this isn't really a good solution.
• Pass the matrix as input or env: this would require the user to manually pass the matrix to the action which is error prone.

[sagikazarmark]

Could be a duplication of #577