Allow global install

[olivier-lacroix]

It would be useful if setup-pixi allowed the installation of global executables (ie like pixi global).

This could either be by referring to a global manifest, or maybe providing a list of package to pass to global install for simpler cases.

This could offer better options than existing specialised GH actions. For instance, setup-cloud does not support caching natively, and take a fair bit of time (20s).

[olivier-lacroix]

This could help address, amongst other use cases, #126