Bug 1917652: Turn on GPU Process for X11 Nightly. r=aosmond

Differential Revision: https://phabricator.services.mozilla.com/D221529

modules/libpref/init/StaticPrefList.yaml

@@ -8542,6 +8542,8 @@
   type: bool
 #if defined(XP_WIN) || defined(MOZ_WIDGET_ANDROID)
   value: true
+#elif defined(MOZ_X11)
+  value: @IS_NIGHTLY_BUILD@
 #else
   value: false
 #endif