[GNC] Build scripts for ubuntu added to the wiki

[david whiting]

I've added scripts for building gnucash in Ubuntu 20.04 LTS and 21.10
to the wiki https://wiki.gnucash.org/wiki/Building_On_Linux in the
section at the end called Distribution Specific Issues, which seemed
to fit in with what was already there. Let me know if you think it
should go elsewhere.

I have decided to put in the whole script for each version of ubuntu,
even though they are very similar to make it easier for people to just
copy the whole script.

David
-- 
David Whiting