Install command line tools without DMG

[kdubau]

You can use this method: https://github.com/timsutton/osx-vm-templates/blob/ce8df8a7468faa7c5312444ece1b977c1b2f77a4/scripts/xcode-cli-tools.sh#L6-L14

This way we don't have to specify the tools dmg.

[TheKevJames]

Thanks for the suggestion! It would certainly be much more convenient to use that method. Not sure when I'll be able to get to it, but I would gladly accept a PR which does this.