ci/nixpkgs-vet: fix random errors

Every now and then, the nixpkgs-vet CI job currently fails with one of:

  error: creating symlink
'/build/.local/share/nix/root/nix/var/nix/gcroots/profiles' ->
'/build/.local/share/nix/root/nix/var/nix/profiles': File exists

  error: SQLite database
'/build/.local/share/nix/root/nix/var/nix/db/db.sqlite' is busy

It's hard to reproduce for me, so just taking a guess with the required
changes.

ci/nixpkgs-vet.nix

@@ -25,6 +25,8 @@ runCommand "nixpkgs-vet"
     env.NIXPKGS_VET_NIX_PACKAGE = nix;
   }
   ''
+    export NIX_STATE_DIR=$(mktemp -d)
+
     nixpkgs-vet --base ${filtered base} ${filtered head}
 
     touch $out