mautrix-signal: Use --prefix instead of --set in wrapper.

Otherwise, options like `systemd.services.<name>.path` have no effect.

An example for this being necessary is to add `ffmpeg` to the path of
a service running `mautrix-signal` in order to decode audio messages.

In general, package wrappers should not make environment variables like
PATH or PYTHONPATH non-overridable.

pkgs/servers/mautrix-signal/default.nix

@@ -47,8 +47,8 @@ python3.pkgs.buildPythonPackage rec {
     " > $out/bin/mautrix-signal
     chmod +x $out/bin/mautrix-signal
     wrapProgram $out/bin/mautrix-signal \
-      --set PATH ${python3}/bin \
-      --set PYTHONPATH "$PYTHONPATH"
+      --prefix PATH : "${python3}/bin" \
+      --prefix PYTHONPATH : "$PYTHONPATH"
   '';
 
   meta = with lib; {