CI: Add ninja-build explicitly to Ubuntu images

On the 18.04 LTS image, this is needed by jhbuild, so make sure it's
included.

Unreviewed.
26 jobs for fix-thorough-tests-failures in 17 minutes and 43 seconds (queued for 6 seconds)
Status Job ID Name Coverage
  Source Check
passed #73818
cppcheck

00:00:36

passed #73795
cpplint

00:00:58

passed #73796
eslint

00:00:40

passed #73793
fedora

00:02:37

failed #73794
cppcheck

00:00:07

 
  Test
passed #73799
no_graphics

00:01:59

passed #73800
no_profiler

00:02:09

passed #73801
no_readline

00:02:09

passed #73797
sanitizer_gcc

00:04:05

passed #73798
with_logging

00:02:26

 
  Manual
manual #73815
allowed to fail manual
armv8
manual #73802
allowed to fail manual
codequality
manual #73803
allowed to fail manual
code_statistics
manual #73817
allowed to fail manual
coverage
manual #73805
allowed to fail manual
fedora_gcc
manual #73811
allowed to fail manual
flatpak
manual #73806
allowed to fail manual
installed_tests
manual #73809
allowed to fail manual
lts
manual #73816
allowed to fail manual
ppc64le
manual #73804
allowed to fail manual
sanitizer_clang
manual #73808
allowed to fail manual
ubuntu_clang
manual #73807
allowed to fail manual
ubuntu_gcc
manual #73810
allowed to fail manual
valgrind
manual #73814
allowed to fail manual
zeal_11
manual #73812
allowed to fail manual
zeal_2
manual #73813
allowed to fail manual
zeal_4