checks: add check for Determinate

This provides a more useful error message than the `/etc/nix/nix.conf`
hash mismatch error that would otherwise occur.

modules/system/checks.nix

@@ -31,6 +31,24 @@ let
     fi
   '';
 
+  determinate = ''
+    if [[ -e /usr/local/bin/determinate-nixd ]]; then
+      printf >&2 '\e[1;31merror: Determinate detected, aborting activation\e[0m\n'
+      printf >&2 'Determinate uses its own daemon to manage the Nix installation that\n'
+      printf >&2 'conflicts with nix-darwin’s native Nix management.\n'
+      printf >&2 '\n'
+      printf >&2 'To turn off nix-darwin’s management of the Nix installation, set:\n'
+      printf >&2 '\n'
+      printf >&2 '    nix.enable = false;\n'
+      printf >&2 '\n'
+      printf >&2 'This will allow you to use nix-darwin with Determinate. Some nix-darwin\n'
+      printf >&2 'functionality that relies on managing the Nix installation, like the\n'
+      printf >&2 '`nix.*` options to adjust Nix settings or configure a Linux builder,\n'
+      printf >&2 'will be unavailable.\n'
+      exit 2
+    fi
+  '';
+
   preSequoiaBuildUsers = ''
     firstBuildUserID=$(dscl . -read /Users/_nixbld1 UniqueID | awk '{print $2}')
     if
@@ -257,6 +275,7 @@ in
 
     system.checks.text = mkMerge [
       (mkIf cfg.verifyMacOSVersion macOSVersion)
+      (mkIf config.nix.enable determinate)
       (mkIf cfg.verifyBuildUsers preSequoiaBuildUsers)
       (mkIf cfg.verifyBuildUsers buildGroupID)
       (mkIf config.nix.enable nixDaemon)