cosmic-session: add orca (screen reader) support

The environment variable `ORCA` is used to specify the path to the
executable binary of orca. If left empty, it defaults to `/usr/bin/orca`
which is not available on NixOS. Setting it to "orca" means that the
executable will be picked up from $PATH.

We can hardcode the path for `ORCA` like we do with `XDP_COSMIC` but
that would introduce an _unnecessary_ dependency, which we would like to
avoid. The support for orca in the COSMIC session on NixOS will be
handled by the NixOS module. Everyone else (non-NixOS-module users)
should have support for orca as long as the executable can be found in
their system's $PATH.

pkgs/by-name/co/cosmic-session/package.nix

@@ -50,7 +50,10 @@ rustPlatform.buildRustPackage (finalAttrs: {
     "target/${stdenv.hostPlatform.rust.cargoShortTarget}"
   ];
 
-  env.XDP_COSMIC = "${xdg-desktop-portal-cosmic}/libexec/xdg-desktop-portal-cosmic";
+  env = {
+    XDP_COSMIC = lib.getExe xdg-desktop-portal-cosmic;
+    ORCA = "orca"; # get orca from $PATH
+  };
 
   passthru = {
     providedSessions = [ "cosmic" ];