Bug 1711850 - Enable MOZ_JXL only for Nightly builds r=aosmond

Differential Revision: https://phabricator.services.mozilla.com/D115512

modules/libpref/init/StaticPrefList.yaml

@@ -5513,11 +5513,13 @@
   value: true
   mirror: always
 
+#ifdef MOZ_JXL
 # Whether we attempt to decode JXL images or not.
 - name: image.jxl.enabled
   type: RelaxedAtomicBool
   value: false
   mirror: always
+#endif
 
 #---------------------------------------------------------------------------
 # Prefs starting with "intl."

toolkit/moz.configure

@@ -560,9 +560,9 @@ set_define("MOZ_AV1", av1)
 option("--disable-jxl", help="Disable jxl image support")
 
 
-@depends("--disable-jxl")
-def jxl(value):
-    if value:
+@depends("--disable-jxl", milestone.is_nightly)
+def jxl(value, is_nightly):
+    if is_nightly and value:
         return True