Cleanup periodically if HOMEBREW_INSTALL_CLEANUP is set.

[MikeMcQuaid]

Follows on from #5165.

Part of #4760.

This will become the default in a later version of Homebrew but has an opt-out through HOMEBREW_NO_INSTALL_CLEANUP.

Also, always cleanup files older than 120 days and set the general default value for "old" logs, casks etc. to 30 days.

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