Adds support for Big Sur

[geerlingguy]

(Note that this doesn't mean it will work, but it does add Big Sur into the script.)

[catap]

@geerlingguy it won't work. Big Sur installer contains SharedSupport.dmg instead of InstallESD.dmg.

[ansonhoyt]

Confirming @catap that it doesn't work for Big Sur...

$ ls /Applications/Install\ macOS\ Big\ Sur.app/Contents/SharedSupport/
SharedSupport.dmg

Looking at these articles, it might not be hard to get it working, but it will require adding a new code path through createISO:

• https://osxdaily.com/2021/05/03/macos-big-sur-dmg-file/
• https://osxdaily.com/2020/12/14/how-create-macos-big-sur-iso/
• https://osxdaily.com/2021/03/24/how-create-macos-big-sur-bootable-install-drive/