CI: Add mono apt repo to fix build
CI: Add mono apt repo to fix build

For whatever reason parts of mono is installed, but the repo is not
configured so there is a version conflict on update:

libglib2.0-cil is already the newest version (2.12.45-0xamarin19+ubuntu1604b1).
libglib2.0-cil set to manually installed.
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 libglib2.0-cil-dev : Depends: libglib2.0-cil (= 2.12.10-6) but 2.12.45-0xamarin19+ubuntu1604b1 is to be installed
E: Unable to correct problems, you have held broken packages.

flatpak/flatpak,flatpak/flatpak,flatpak/flatpak,flatpak/flatpak,flatpak/flatpak