fix build with assistant disabled and webapp enabled

The webapp modules cannot build with the assistant disabled, so make the
webapp be under the assistant build flag.

Sponsored-by: Jarkko Kniivilä on Patreon

stack.yaml

@@ -1,10 +1,10 @@
 flags:
   git-annex:
     production: true
-    assistant: true
+    assistant: false
     pairing: true
     torrentparser: true
-    webapp: true
+    webapp: false
     magicmime: false
     dbus: false
     debuglocks: false