formula_installer: handle unexpected .brew presence/absence

[EricFromCanada]

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This fixes two complementary errors I've seen recently:

• formula_installer.rb: when running brew reinstall, reinstall.rb checks for the existing keg via its opt prefix and backs it up to #{path}.reinstall if it exists. But if for some reason that opt prefix symlink is missing, the installer will happily overwrite the existing keg in-place… until it attempts to create the ".brew" folder, which throws Error: File exists @ dir_s_mkdir - /usr/local/Cellar/<formula>/<version>/.brew. Swapping mkpath for mkdir avoids the error.
• formula_cellar_checks.rb: some bottles that have been around for a while (e.g. yajl) lack the ".brew" directory, which causes check_cpuid_instruction to complain with Error: No such file or directory @ rb_sysopen - /usr/local/opt/<formula>/.brew/<formula>.rb when installing. I updated it to a) check for an Intel CPU first, and b) check for the full path to the .brew/<formula>.rb file before attempting to read it.

[BrewTestBot]

Review period will end on 2022-04-13 at 21:56:31 UTC.

[Bo98]

These make sense to me.

[MikeMcQuaid]

Looks good but optional DRY up.

[MikeMcQuaid]

Thanks again @EricFromCanada!

[BrewTestBot]

Review period skipped due to critical label.