owncast: fix ffmpeg issues with systemd

The `bash` and `which` wrapper dependencies were mistakenly removed last
package update.
The package works when run manually since you already have bash and
which in your path. Only when isolated in systemd does the issue emerge.

pkgs/servers/owncast/default.nix

···
 , buildGoModule
 , fetchFromGitHub
 , nixosTests
+, bash
+, which
 , ffmpeg
 , makeBinaryWrapper
 }:
···
 
   postInstall = ''
     wrapProgram $out/bin/owncast \
-      --prefix PATH : ${lib.makeBinPath [ ffmpeg ]}
+      --prefix PATH : ${lib.makeBinPath [ bash which ffmpeg ]}
   '';
 
   installCheckPhase = ''