voxtype: fix output mode paste

Voxtype requires the command `cat` from
`coreutils` when using the output mode `paste`,
but fails with the current configuration because
it cannot find `cat`.

```
WARN paste (clipboard + keystroke) failed: Text
injection failed: wl-copy exited with error,
trying next
```

This commit adds `coreutils` to the runtime path
to resolve the path to `cat`.

modules/services/voxtype.nix

@@ -141,7 +141,10 @@ in
       Service =
         let
           runtimePath = makeBinPath (
-            [ pkgs.which ]
+            [
+              pkgs.coreutils
+              pkgs.which
+            ]
             ++ optionals (cfg.x11.display != null) [ pkgs.xclip ]
             ++ optionals (cfg.wayland.display != null) [
               pkgs.wl-clipboard

tests/modules/services/voxtype/basic-configuration.nix

@@ -31,7 +31,7 @@
       WantedBy=default.target
 
       [Service]
-      Environment=PATH=@which@/bin:@wl-clipboard@/bin:@wtype@/bin
+      Environment=PATH=/nix/store/00000000000000000000000000000000-coreutils/bin:@which@/bin:@wl-clipboard@/bin:@wtype@/bin
       Environment=XDG_RUNTIME_DIR=%t
       Environment=WAYLAND_DISPLAY=wayland-1
       Environment=VOXTYPE_TEST_ENV=1