Bump nixpkgs rev (#25361)

* Bump nixpkgs revision

* Update nix-direnv

* Remove extra nix functionalities

Both nix-direnv and legacy command nix-shell fail. Reverting to
flakes-only commands.

* Pin nixpkgs per suggestion

See https://github.com/space-wizards/space-station-14/pull/25361#discussion_r1494196038

* Revert nix-direnv removal

* Add python3 to shell.nix

flake.lock

@@ -20,11 +20,11 @@
     },
     "nixpkgs": {
       "locked": {
-        "lastModified": 1706213598,
-        "narHash": "sha256-Toz3HEHeq6Esr5uDOMel8BiGSa94gj+og3Yz4YEgjYI=",
+        "lastModified": 1708210246,
+        "narHash": "sha256-Q8L9XwrBK53fbuuIFMbjKvoV7ixfLFKLw4yV+SD28Y8=",
         "owner": "NixOS",
         "repo": "nixpkgs",
-        "rev": "cda0e75a0bd7cf05bd3e40658c163e4f8f376b7b",
+        "rev": "69405156cffbdf2be50153f13cbdf9a0bea38e49",
         "type": "github"
       },
       "original": {