nixos/profiles/base: install vim w/nix-syntax plugin

Considering that you most likely edit Nix code in the installer, that
seems like a useful thing.

The size of the ISO I got from

    nix-build nixos/release.nix -A iso_minimal.x86_64-linux

is still at 877M.

(cherry picked from commit 0b5a0cbc69)

nixos/modules/profiles/base.nix

@@ -20,7 +20,13 @@
     pkgs.mkpasswd # for generating password files
 
     # Some text editors.
-    pkgs.vim
+    (pkgs.vim.customize {
+      name = "vim";
+      vimrcConfig.packages.default = {
+        start = [ pkgs.vimPlugins.vim-nix ];
+      };
+      vimrcConfig.customRC = "syntax on";
+    })
 
     # Some networking tools.
     pkgs.fuse