updateiconcache: Sort list of entries
because filesystem readdir order is indeterministic.
Without this patch, building openSUSE's balsa package had variations between builds in /usr/share/balsa/icon-theme.cache
patch inspired by https://bugzilla.gnome.org/show_bug.cgi?id=786528
Alternatively, one could also sort in write_file, but would have to handle key+value pairs and make sure that all previous processing happens independent of filesystem order.
Interestingly, https://tests.reproducible-builds.org/debian/rb-pkg/unstable/amd64/balsa.html lists Debian's balsa-2.5.5 package already as reproducible with libgtk-3-0 (= 3.22.30-2) though I cannot find patches for this.