buildMozillaMach: pin icu at 73 for now

Pending https://bugzilla.mozilla.org/show_bug.cgi?id=1862601.

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

@@ -62,8 +62,8 @@ in
 , glib
 , gnum4
 , gtk3
-, icu
 , icu72
+, icu73
 , libGL
 , libGLU
 , libevent
@@ -487,7 +487,9 @@ buildStdenv.mkDerivation {
   ]
   # icu73 changed how it follows symlinks which breaks in the firefox sandbox
   # https://bugzilla.mozilla.org/show_bug.cgi?id=1839287
-  ++ [ (if (lib.versionAtLeast version "115") then icu else icu72) ]
+  # icu74 fails to build on 127 and older
+  # https://bugzilla.mozilla.org/show_bug.cgi?id=1862601
+  ++ [ (if (lib.versionAtLeast version "115") then icu73 else icu72) ]
   ++ [ (if (lib.versionAtLeast version "116") then nss_latest else nss_esr/*3.90*/) ]
   ++ lib.optional  alsaSupport alsa-lib
   ++ lib.optional  jackSupport libjack2

pkgs/applications/networking/mailreaders/thunderbird/packages.nix

@@ -1,4 +1,4 @@
-{ stdenv, lib, buildMozillaMach, callPackage, fetchurl, fetchpatch, nixosTests, icu, fetchpatch2, config }:
+{ stdenv, lib, buildMozillaMach, callPackage, fetchurl, fetchpatch, nixosTests, icu73, fetchpatch2, config }:
 
 rec {
   thunderbird = thunderbird-115;
@@ -40,7 +40,7 @@ rec {
 
     pgoSupport = false; # console.warn: feeds: "downloadFeed: network connection unavailable"
 
-    icu = icu.overrideAttrs (attrs: {
+    icu73 = icu73.overrideAttrs (attrs: {
       # standardize vtzone output
       # Work around ICU-22132 https://unicode-org.atlassian.net/browse/ICU-22132
       # https://bugzilla.mozilla.org/show_bug.cgi?id=1790071