Build appimage on Ubuntu 20.04

[serock]

Besides changing the versions of Ubuntu and GNAT, the only other change is to get the build to use GCC 10 instead of GCC 9. The $HOME/.local/bin directory is already in the PATH environment variable before the /usr/bin directory. The changes create the $HOME/.local/bin directory and create a gcc symbolic, in that directory, that points to gcc-10.

Fixes #1511.

[mosteo]

Thanks, @serock