FIX: Errno::EXDEV when across filesystem boundaries (PR #13976)

ref https://bugs.ruby-lang.org/issues/12615

        W: rake aborted!
        W: Errno::EXDEV: Invalid cross-device link @ rb_file_s_rename - (/app/tmp/adeeee62504de67238341871bda1aaf0, /app/app/assets/javascripts/plugins/discourse-cakeday.js.e
rb)
        W: lib/discourse.rb:65:in `rename'
        W: lib/discourse.rb:65:in `atomic_write_file'
        W: /app/lib/plugin/instance.rb:683:in `activate!'
        W: lib/discourse.rb:246:in `block in activate_plugins!'
        W: lib/discourse.rb:243:in `each'
        W: lib/discourse.rb:243:in `activate_plugins!'

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