ci: Include details of machine ID in CI output

Include the size of the `machine-id` file, but not the value itself as
that is sensitive for non-throwaway machines. What’s most useful for
debugging CI problems is knowing whether, and where, the `machine-id` is
set.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

.gitlab-ci/show-execution-environment.sh

@@ -9,3 +9,5 @@ setpriv --dump || :
 ulimit -a || :
 cat /proc/self/status || :
 cat /proc/self/mountinfo || :
+stat /etc/machine-id || :
+stat /var/lib/dbus/machine-id || :