setup-user: use github.com/moby/sys/user

[neersighted]

• Closes #131
• Closes #130

Use the new github.com/moby/sys/user module (and clean up the dependency tree) to remove the runc dependency and reduce the possibility of future conflicts resulting from dependencies.

[neersighted]

Oh, shoot, I kept that commit message from runc. That should be dropped here.

[tianon]

Ah, that makes more sense :smile:

Would you like to update, or should I instead just squash and merge? (I have no strong preference either way)

[eshafaq1]

TY @neersighted