Do not try to update github-pages on pull_request

[rffontenelle]

• This fix issue:

PR Description:

When a pull request is created, the github-pages.yml workflow file builds the docs and tries to update the published docs. It fails with:

remote: Permission to archlinux/archinstall.git denied to github-actions[bot].
fatal: unable to access 'https://github.com/archlinux/archinstall.git/': The requested URL returned error: 403
Error: Action failed with "The process '/usr/sbin/git' failed with exit code 128"

See this example.

This adds condition to not run "Deploy to GitHub Pages" step on pull requests

Tests and Checks

• I have tested the code!
  

[svartkanin]

I think that makes sense, thanks for the fix