Switch to flake-based build on nix-builder for nightly rust.

nix/configuration/flake.lock

@@ -162,6 +162,27 @@
         "type": "github"
       }
     },
+    "nix_builder": {
+      "inputs": {
+        "nixpkgs": [
+          "nixpkgs"
+        ],
+        "rust-overlay": "rust-overlay_2"
+      },
+      "locked": {
+        "lastModified": 1783278211,
+        "narHash": "sha256-/1u+MIQIge+cNPukQoK4Jp8nLuFZRbx4U+MyyxG0RpM=",
+        "ref": "refs/heads/main",
+        "rev": "9281ba7e10d362d4edb489d0df0e78936dfe7b58",
+        "revCount": 32,
+        "type": "git",
+        "url": "https://code.fizz.buzz/talexander/nix_builder.git"
+      },
+      "original": {
+        "type": "git",
+        "url": "https://code.fizz.buzz/talexander/nix_builder.git"
+      }
+    },
     "nixpkgs": {
       "locked": {
         "lastModified": 1780749050,
@@ -242,6 +263,7 @@
         "disko": "disko",
         "impermanence": "impermanence",
         "lanzaboote": "lanzaboote",
+        "nix_builder": "nix_builder",
         "nixpkgs": "nixpkgs",
         "nixpkgs-google": "nixpkgs-google"
       }
@@ -266,6 +288,27 @@
         "repo": "rust-overlay",
         "type": "github"
       }
+    },
+    "rust-overlay_2": {
+      "inputs": {
+        "nixpkgs": [
+          "nix_builder",
+          "nixpkgs"
+        ]
+      },
+      "locked": {
+        "lastModified": 1779506148,
+        "narHash": "sha256-OffE5yuMrSW71zIJNLvtl9MCO6WTAHEjlFPUCvkd/QM=",
+        "owner": "oxalica",
+        "repo": "rust-overlay",
+        "rev": "d9973e2ab49747fada06ebbe26cda27eb0220cf1",
+        "type": "github"
+      },
+      "original": {
+        "owner": "oxalica",
+        "repo": "rust-overlay",
+        "type": "github"
+      }
     }
   },
   "root": "root",