Bug 1840480 - [css-properties-values-api] Enable on nightly. r=zrhoffman

Even though it still requires significant work, this allows us to keep
track of progress more properly in wpt.fyi.

Since we expect implementation progress to improve quite a bit in H2,
and this shouldn't be an stability hazard, this seems fine unless the
missing bits cause big compat issues.

Differential Revision: https://phabricator.services.mozilla.com/D182111

modules/libpref/init/StaticPrefList.yaml

@@ -8765,7 +8765,7 @@
 # Whether Properties and Values is enabled
 - name: layout.css.properties-and-values.enabled
   type: RelaxedAtomicBool
-  value: false
+  value: @IS_NIGHTLY_BUILD@
   mirror: always
   rust: true