Bug 1813145 - Enable AVIF sequences on nightly. r=tnikkel

Differential Revision: https://phabricator.services.mozilla.com/D168629

modules/libpref/init/StaticPrefList.yaml

@@ -7120,7 +7120,7 @@
 # Whether to allow decoding of animated AVIF sequences.
 - name: image.avif.sequence.enabled
   type: RelaxedAtomicBool
-  value: false
+  value: @IS_NIGHTLY_BUILD@
   mirror: always
 
 # Whether AVIF files containing sequences should be animated even when the