slack: 4.0.2 -> 4.1.2

Also:
- Add missing dependency on libpulseaudio
- Extract the deb in the build folder and move only the files we're
  interested in to $out
- Use the regular builder, but turn off the patchELF hook
- Move dpkg from buildInputs to nativeBuildInputs, since it's only
  used at build time