integration_test: only run on system Ruby.

[MikeMcQuaid]

There appear to be random, seemingly impossible to debug issues with running integration tests on portable Ruby. Instead of confusing contributors when these will be run on CI anyway: let's just skip them by default (like we do with --online for online tests anyway).

Note to self: ensure that CI is definitely using system Ruby before this is merged...

• 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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[Bo98]

The crash specifically happens when the launched Ruby process launches another Ruby process (e.g. like in brew ruby, brew test, brew install etc.) and that Ruby accesses $LOAD_PATH in any manner (Rubygems does $LOAD_PATH.each_with_index but puts $LOAD_PATH also crashes).

[MikeMcQuaid]

that Ruby accesses $LOAD_PATH in any manner (Rubygems does $LOAD_PATH.each_with_index but puts $LOAD_PATH also crashes).

Good catch, this part is news to me.