signal-desktop: use autoPatchelfHook, wrap properly (#77850)

Most prominently we don't use `phases` because of https://github.com/NixOS/nixpkgs/issues/28910.
This is also problematic when using wrapGAppsHook.
In order to use wrapGAppsHook's automatic wrapping
(this was done manually before because there was no fixupPhase)
we need to install signal at lib/Signal instead of just into libexec.
That's because it would try to wrap .so files.