cleanup: fix input handling

[maxim-belkin]

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--prune flag expects an integer and when anything else is specified, it gets converted to 0 (args.prune&.to_i).
That implicit conversion to 0 made the integration test for cleanup that uses --prune=all to pass.

[Bo98]

--prune=all being supported is intentional, and used by others: https://github.com/search?l=Shell&q=%22brew+cleanup+--prune%3Dall%22&type=Code, as well as one of our own ops scripts.

[maxim-belkin]

--prune=all being supported is intentional

all is not handled in any way -- any string has the same effect and is an equivalent to --prune=0. I can add support for "all" and revert changes to tests, if you think that's going to be useful.

[Bo98]

There used to be an explicit check for it but it disappeared in some refactoring - I don't think intentionally.