spin/push: clean up registry credentials post-run

[vdice]

Add a post-run step that removes login credentials (from spin registry login) automatically when using the spin/push action.
(Inspired by the docker login action.)

The spin CLI doesn't yet have a spin registry logout command, though this might inspire the addition :) In the meantime, I think we'd want to remove the directory (or file) containing the credentials (eg /home/runner/fermyon or /home/runner/fermyon/registry-auth.json, respectively.)