unpack_strategy: fix unpack Dir.mktmpdir group

[cho-m]

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Otherwise the files will be unpacked with group wheel on macOS which gets copied over when running cp -al.

This used to be implicitly "fixed" due to cp -pR usage which would recreate the files.

After #18497, files that are copied via cp -al retain wheel group, which isn't what we want.

Was exploring another alternative in #18518 but this seems like cleaner option.

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Probably should move the logic to utils/ or somewhere else that can be shared to avoid code duplication. Perhaps a follow up as would be good to avoid current bottles getting installed as wheel group.

[MikeMcQuaid]

Thanks @cho-m! Good to go once error message improved.

[cho-m]

Also exploring a way to avoid duplication in #18560 by modifying Mktemp. If that goes in, can just run Mktemp.new("homebrew-unpack").run(chdir: false)

[Bo98]

Will merge this for now given it's regression fix.