utils/gems: prioritise Homebrew's chosen Ruby

[Bo98]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew tests with your changes locally?

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As seen in #5561 (comment), Ruby in other locations like rbenv may be used when installing gems. At the very least, this can mean installing native extensions to a place that won't match what Homebrew's chosen Ruby actually uses. At worst, we get hard to debug errors.

The reason other Rubys were being picked was because ~/.gem/2.6.0/bin was prioritised over Homebrew's chosen Ruby bin directory. This may mean that ~/.gem/2.6.0/bin/bundle is run instead.

This is a separate issue to the TMPDIR one.

[MikeMcQuaid]

Makes sense, thanks @Bo98.