Add lib-stripped artifact to build pipeline

[coloursofnoise]

Generates an artifact using mono-cil-strip of the Celeste libs post-patch
For use by mods as part of their build pipelines

[WEGFan]

Maybe also split the process of patching to an individual one so we can add a patch check for commits across all versions of Celeste? (although only FNA is available since it's on Ubuntu)

[coloursofnoise]

Maybe also split the process of patching to an individual one so we can add a patch check for commits across all versions of Celeste? (although only FNA is available since it's on Ubuntu)

Definitely in favour of this, including moving the patching to happen before creating artifacts, since if it fails patching then we probably shouldn't ship the build 😅
For this PR though I'd rather just get the lib-stripped artifact out and usable, and I'll work on a second PR for proper patch checks at a later date.