flake: use rust-overlay in dev shell

This allows `niri-visual-tests` to still be built and run in the dev
shell where it's necessary, as well as brings back the nightly `rustfmt`
used by the project

We can't use `fenix` again though as it doesn't wrap `ld` like nixpkgs
and rust-overlay do; without it, the way we link `dlopen()`'d libraries
breaks

flake.lock

@@ -34,7 +34,28 @@
     "root": {
       "inputs": {
         "nix-filter": "nix-filter",
-        "nixpkgs": "nixpkgs"
+        "nixpkgs": "nixpkgs",
+        "rust-overlay": "rust-overlay"
+      }
+    },
+    "rust-overlay": {
+      "inputs": {
+        "nixpkgs": [
+          "nixpkgs"
+        ]
+      },
+      "locked": {
+        "lastModified": 1727663505,
+        "narHash": "sha256-83j/GrHsx8GFUcQofKh+PRPz6pz8sxAsZyT/HCNdey8=",
+        "owner": "oxalica",
+        "repo": "rust-overlay",
+        "rev": "c2099c6c7599ea1980151b8b6247a8f93e1806ee",
+        "type": "github"
+      },
+      "original": {
+        "owner": "oxalica",
+        "repo": "rust-overlay",
+        "type": "github"
       }
     }
   },