style: chdir before running rubocop

[Bo98]

This fixes longstanding issues when the working directory is incorrect, including incorrect globbing of include/excludes and spurious warnings such as:

Tip: Based on detected gems, the following RuboCop extension libraries might be helpful:
  * rubocop-minitest (https://rubygems.org/gems/rubocop-minitest)

You can opt out of this message by adding the following to your config (see https://docs.rubocop.org/rubocop/extensions.html#extension-suggestions for more options):
  AllCops:
    SuggestExtensions: false

[Rylan12]

This solves the issue for me, thank you!

[apainintheneck]

I haven't seen the old .rbi file problems and RuboCop extension warnings recently but it still makes sense to keep this consistent regardless of the working directory.

• Related Issue: brew style includes .rbi files when run from tap #15472

[MikeMcQuaid]

Makes sense, thanks @Bo98!