chore: add nix stuff to gitignore

Author: TheButlah

.gitignore

@@ -30,4 +30,8 @@ rust/
 !examples/serde-rs/ta/Cargo.lock
 !examples/tls_client-rs/ta/Cargo.lock
 !examples/tls_server-rs/ta/Cargo.lock
-!projects/web3/eth_wallet/ta/Cargo.lock
+!projects/web3/eth_wallet/ta/Cargo.lock
+
+# NixOS
+.direnv/
+.envrc