buildMozillaMach: pin icu to 72

Fixes an issue where the timezone would not be correctly read, due to a
behavior change in icu 73, where the path to the timezone is now read
using realpath instead of readlink.

This leads lookups for /etc/localtime into the nix store, instead of
into /etc/zoneinfo, which gets blocked by sandboxing.

Reported upstream at https://bugzilla.mozilla.org/show_bug.cgi?id=1839287.

pkgs/applications/networking/browsers/firefox/common.nix

@@ -54,7 +54,7 @@
 , glib
 , gnum4
 , gtk3
-, icu
+, icu72
 , libGL
 , libGLU
 , libevent
@@ -440,7 +440,9 @@ buildStdenv.mkDerivation ({
     freetype
     glib
     gtk3
-    icu
+    # icu73 changed how it follows symlinks which breaks in the firefox sandbox
+    # https://bugzilla.mozilla.org/show_bug.cgi?id=1839287
+    icu72
     libffi
     libGL
     libGLU