nix/vm: isolate it a bit more

I personally don't like that nixos-shell inherits a ton of stuff from the
host by default, even mounting my home directory as r/w! I imagine I'm not
the only one with this opinion, so let's put a stop to it by default.

Signed-off-by: Winter <[e-mail]>

nix/vm.nix

···
         pkgs,
         ...
       }: {
+        nixos-shell = {
+          inheritPath = false;
+          mounts = {
+            mountHome = false;
+            mountNixProfile = false;
+          };
+        };
         virtualisation = {
           memorySize = 2048;
           diskSize = 10 * 1024;