Install gtk-sharp2 package for tests run on ubuntu-latest.
Install gtk-sharp2 package for tests run on ubuntu-latest.

This seems to have been previously included when the job ran on Ubuntu
18.04.5.  Now that it has been upgraded to Ubuntu 20.04.2, the gtk-sharp
libraries appear to be missing.

mnaiman/duplicati,duplicati/duplicati,mnaiman/duplicati,mnaiman/duplicati,duplicati/duplicati,duplicati/duplicati,mnaiman/duplicati,duplicati/duplicati,mnaiman/duplicati,duplicati/duplicati