standalone: Fix extraSpecialArgs (#799)

This commit fixes the usage of `extraSpecialArgs` for standalone nixvim
installations and adds a test to ensure this.

Fixes #779

tests/extra-args.nix

@@ -0,0 +1,26 @@
+{
+  makeNixvimWithModule,
+  pkgs,
+}: let
+  generated = makeNixvimWithModule {
+    module = {importedArgument, ...}: {
+      extraConfigLua = ''
+        -- importedArgument=${importedArgument}
+      '';
+    };
+    extraSpecialArgs = {
+      importedArgument = "foobar";
+    };
+  };
+in
+  pkgs.runCommand "special-arg-test" {
+    printConfig = "${generated}/bin/nixvim-print-init";
+  } ''
+    config=$($printConfig)
+    if ! "$printConfig" | grep -- '-- importedArgument=foobar'; then
+      echo "Missing importedArgument in config"
+      exit 1
+    fi
+
+    touch $out
+  ''