brew.rb: remove gem setup for stackprof

[Bo98]

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GEM_HOME and GEM_PATH are already set here, so setting up the gem environment here is not necessary and causes issues due to the assumption that standalone/load_path is run first. This fixes stackprof not working.

This could be refined further so that we don't allow GEM_* bleeding and instead insert the relevant directory into the load path via a Ruby command arg (like we do for intergration tests), to ensure better consistency in runtime behaviour between brew prof runs and regular env-filtered runs. That is however an extension of this PR rather than an alternative solution.

Closes #11799

[BrewTestBot]

Review period will end on 2021-08-12 at 01:21:59 UTC.

[Rylan12]

I don't know a ton about gem loading and the like but this seems to make sense and works fine locally

[MikeMcQuaid]

Great, thanks @Bo98!

[BrewTestBot]

Review period skipped due to critical label.