Add threads argument

[omus]

Currently enabling threads using this action require you to set the enviromental variable JULIA_NUM_THREADS. Although this does work there is no way to use this variable to specify "auto" on Julia version < 1.7. It would be great to have support for a threads argument that lets you specify julia --threads which would allow for use of "auto" on earlier Julia versions.

[DilumAluthge]

I wonder if we should just have an input named cmd_line_flags that lets the user pass whatever flags they want.

[SaschaMann]

I wonder if we should just have an input named cmd_line_flags that lets the user pass whatever flags they want.

I have thought about that before but I kinda don't see the point of even using this action if you need to customise your build script that much. At that point you might as well just write whatever you're trying to run directly into the workflow.

That said, I don't have a strong opinion on it.