ci: switch to Ubuntu 20.04

ci: switch to Ubuntu 20.04

Ubuntu 18.04 is now rather old.
Besides, other entities in our CI are also testing this distribution.

Switch to a newer Ubuntu release and benefit from more recent
tool(chain)s: for example, net/cnxk now builds fine and can be
re-enabled.

Note: Ubuntu 18.04 and 20.04 seem to preserve the same paths for the ARM
and PPC cross compilation toolchains, so we can use a single
configuration file (with the hope, future releases of Ubuntu will do the
same).

Signed-off-by: David Marchand <david.marchand@redhat.com>
Acked-by: Aaron Conole <aconole@redhat.com>
Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>