tox.ini: it's OK if libgap-dev is missing on ubuntu-focal

Author: orlitzky

tox.ini

@@ -206,6 +206,7 @@ setenv =
     #
     # https://hub.docker.com/_/ubuntu?tab=description
     # as of 2023-05, latest=jammy=22.04, rolling=lunar=23.04, devel=mantic=23.10
+    # ubuntu-focal does not have libgap-dev
     #
     ubuntu:         SYSTEM=debian
     ubuntu:         BASE_IMAGE=ubuntu
@@ -221,6 +222,7 @@ setenv =
     ubuntu-bionic:    BASE_TAG=bionic
     ubuntu-bionic:                             IGNORE_MISSING_SYSTEM_PACKAGES=yes
     ubuntu-focal:     BASE_TAG=focal
+    ubuntu-focal:                              IGNORE_MISSING_SYSTEM_PACKAGES=yes
     ubuntu-jammy:     BASE_TAG=jammy
     ubuntu-lunar:     BASE_TAG=lunar
     ubuntu-mantic:    BASE_TAG=mantic