CI: Skip certain fontconfig leaks

For some reason, the Debian CI setup leaks fontconfig allocations in a
way which it does not for me on Fedora. On the assumption that this has
been fixed between our older CI Debian and Fedora 36, suppress the leak
warning, because we do already call FcFini() which should free it.

Signed-off-by: Daniel Stone <daniels@collabora.com>

.gitlab-ci/leak-sanitizer.supp

@@ -6,3 +6,6 @@ leak:cairo_text_extents
 
 # Pango thread-global state (not destroyable?)
 leak:pango_language_get_default
+
+# This leaks in Debian's fontconfig/Xwayland setup?
+leak:FcConfigSubstituteWithPat