You can build Sigil out of the box on Ubuntu. Have you tried building your own using our Linux build docs?
The child process error from Eddy is common according to a google search. If Ubuntu 22.04 is LTS why are they not updating Sigil? Where is the actual Sigil build issue on that platform?
Update: Found this which means Sigil-1.9.20 is progressing okay.
https://launchpad.net/ubuntu/+source/sigil/+changelog