Use Ubuntu 20.04 explicitly.
Use Ubuntu 20.04 explicitly.

nickerso/libcellml,nickerso/libcellml,cellml/libcellml,nickerso/libcellml,hsorby/libcellml,hsorby/libcellml,nickerso/libcellml,cellml/libcellml,cellml/libcellml,cellml/libcellml,hsorby/libcellml,hsorby/libcellml