gitlab-ci.yml: Test build on CentOS 7

We should also test our build with older versions of Bash and Gcc (at
least the versions which we still want to support). CentOS 7 should be
a reasonable base for such tests.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20200731091548.8302-1-thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>