feat(CI): Add workflow for building the Steam depot

[tibetiroka]

Adds a workflow for building the Steam depot. This is currently triggered manually.

I will test that it creates the depot with the correct contents.

[tibetiroka]

Works. The generated steam depot can be checked in this workflow.

[tibetiroka]

By the way, I could move this over to the release workflow in the main repo. Would that be more convenient?

[warp-core]

That'd be a question for @MCOfficer since he's the one that does that.
Since we don't run any workflows for a release over here, it certainly seems like it'd be more convenient to have it over there where we run that workflow anyway.
But, it might be odd/confusing for something related to this package to be in that repository. And, it's not really a big inconvenience to come over here and run a workflow.

[MCOfficer]

IMO the arguments for here or there kinda balance each other out, so let's pick the option with less (code) complexity and have it be here.