Expand .xip on the destination volume

After PR #126, xcode install supports a custom path for Xcode's installation, but this path isn't directly used for the .xip expansion, using instead the same directory where it’s downloaded (~/Library/Application Support/com.robotsandpencils.xcodes/)[^1]. If this custom installation path is to a different volume than the .xip, this means that:

• Xcode.app must be first expanded and then moved to another volume, a relatively slow operation given its massive size.
• This installation requires double the free space than on a same-volume installation, because Xcode is first copied to the destination volume and only when it has finished it’s deleted from the original volume.

So I think it makes sense to expand the .xip directly on the final volume where Xcode will be installed (probably on the /.TemporaryItems folder).

Would you accept a pull request for this? Thank you!

[^1]: I assume this is done to avoid having a partial Xcode installation on the destination folder, which sounds reasonable to me.

Start at the xcode install flow changed by PR #126 and trace where the downloaded .xip is expanded and where the custom installation path is applied. The change is complete when expansion occurs directly on the destination volume, avoiding the intermediate copy and extra free-space requirement; verify behavior across different volumes.