cleanup.rb: ensure cache exists before touching .cleaned

[EricFromCanada]

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Make sure that HOMEBREW_CACHE exists before creating the .cleaned file. Might fix occasional "Error: No such file or directory @ rb_sysopen - /Users/brew/Library/Caches/Homebrew/.cleaned" errors for bottle builds involving lots of dependents. (xref #5510)

One might note that in the aforelinked job, only the Mojave runner was repeatedly deciding to run Formulae#cleanup_during! after every dependent after a certain point, which suggests that something's awry with the free space detection on that OS or runner.

[MikeMcQuaid]

Good catch, thanks @EricFromCanada!