fix: ensure that .netlify is a directory and not a file or broken symlink

[danez]

Summary

Fixes https://app.bugsnag.com/netlify/netlify-build/errors/63b6daab56c5cf00082ee11f?event_id=63b6daab00a2f84cc76b0000&i=sk&m=nw

Also in the build log of the affected build I can see:

3:10:59 PM: mkdir: cannot create directory ‘/opt/build/repo/.netlify’: File exists
3:10:59 PM: Started restoring cached build plugins
3:10:59 PM: mv: failed to access '/opt/build/repo/.netlify/plugins': Not a directory
3:10:59 PM: Finished restoring cached build plugins

It seems some repos have .netlify as a file. For example: https://github.com/MailTape/MailTape.github.io/blob/master/.netlify Is this an old format of the config file? What is that? To me this looks like the CLI global config file.

This change ensures that if .netlify is not a directory we remove it before we try to create it.

Alternatively, we could also error the build if the file is present?

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