pantheon.switchboard-with-plugs: fix wrapping

Since #81475 this caused the wrapper to be empty of entries from
wrapGAppsHook because the wrapGAppsHook function doesn't add
them anymore, and was moved to gappsWrapperArgsHook. Instead
of just running that in postBuild it's more future proof to make this
use stdenv.mkDerivation because we want to mess around with the
generic builder.