snapcraft: Try installing postcss again

Revert "snap: Skip installing postcss due to failure on build server"

This reverts commit 66f688f712.

snapcraft.yaml

@@ -57,8 +57,16 @@ parts:
 
   node:
     plugin: nodejs
-    prime:
+    node-packages: [postcss-cli]
+    filesets:
+      node:
         - bin/node
+      postcss:
+        - bin/postcss
+        - lib/node_modules/postcss-cli/*
+    prime:
+      - $node
+      - $postcss
 
   pygments:
     plugin: python