Use injected `XCODE_INSTALL_CACHE_DIR`variable if exist.

[makesource]

Thank you for great library 😀

Issue

We can specify the downloaded xcode folder with ENV["XCODE_INSTALL_CACHE_DIR"] But when downloading not cached Xcode version, the file stored the default directory #{ENV['HOME']}/Library/Caches/XcodeInstall. So to work local cache properly, I have to move xip file after download job done.

Proposal

If user injected XCODE_INSTALL_CACHE_DIR variable, the install job executed under this path.