Moves Nix.gitignore to root because popular

The Nix repo(https://github.com/NixOS/nix) has over 14k stars and over 1k
clones. I believe this project is widely used and popular enough to move
to the root now.

Nix.gitignore

@@ -0,0 +1,6 @@
+# Ignore build outputs from performing a nix-build or `nix build` command
+result
+result-*
+
+# Ignore automatically generated direnv output
+.direnv