cleanup: don't remove portable ruby on periodic cleanups.

[MikeMcQuaid]

Otherwise we end up deleting the running Ruby version.

Fixes #5707.

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[Amit-PivotalLabs]

Thanks @MikeMcQuaid. Do you have any suggestions for anyone who used brew before this fix and lost portable ruby?

[MikeMcQuaid]

@Amit-PivotalLabs Why do you need portable Ruby?