Simplify auto spotless workflow

[trask]

Also should fix the edge case where PR is coming from main branch on fork: #1855 (comment)

The problem before was that there was already a main branch checked out locally, but now it checks out remote branch directly.

[laurit]

I have a vague memory that as a maintainer I was not allowed to push into the main branch of the fork. If that is true then probably this won't work anyway.

[trask]

I have a vague memory that as a maintainer I was not allowed to push into the main branch of the fork. If that is true then probably this won't work anyway.

I just tested pushing to #1855 and seems to work: #1855 (comment)