Bug 1456995 - enable Service Workers' parent-intercept mode on Nightly r=asuth

Differential Revision: https://phabricator.services.mozilla.com/D43170

--HG--
extra : moz-landing-system : lando

modules/libpref/init/StaticPrefList.yaml

@@ -2103,7 +2103,7 @@
 # Note, this is not currently safe to use for normal browsing yet.
 - name: dom.serviceWorkers.parent_intercept
   type: bool
-  value: false
+  value: @IS_NIGHTLY_BUILD@
   mirror: never
 
 - name: dom.serviceWorkers.testing.enabled