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.
Wingpanel was designed firstly as an indicator renderer,
and as such just a container for indicators that are distributed
outside itself. Being able to control which and each indicator with
`indicators` is confusing, ideally each of the default indicators
would be shipped with wingpanel itself. I don't see how this
kind of extensibility would be useful to a user so we're going
to append to the expected defaults. The `useDefaultIndicators`
argument is there to development test a single indicator/s.
