Welcome to The New Switchroot L4T Ubuntu Linux Repo.

Import this key: https://newrepo.switchroot.org/pubkey

Add this line to your /etc/apt/sources.list (or somewhere in /etc/apt/sources.list.d): 
deb https://newrepo.switchroot.org/ switchroot unstable

If you're on 20.04 (Focal Fossa) also add this: 
deb https://newrepo.switchroot.org/ switchroot focal

If you're on 20.10 (Groovy Gorilla) also add this: 
deb https://newrepo.switchroot.org/ switchroot groovy

Enjoy,
Ave