Update to Nix master

The point of this branch is to always track Nix master, so we are
proactively ready to upgrade to the next Nix release when it is ready.

Flake lock file updates:

• Updated input 'nix':
    'github:NixOS/nix/50f8f1c8bc019a4c0fd098b9ac674b94cfc6af0d' (2023-11-27)
  → 'github:NixOS/nix/c3827ff6348a4d5199eaddf8dbc2ca2e2ef46ec5' (2023-12-07)
• Added input 'nix/libgit2':
    'github:libgit2/libgit2/45fd9ed7ae1a9b74b957ef4f337bc3c8b3df01b5' (2023-10-18)

flake.lock

@@ -16,6 +16,22 @@
         "type": "github"
       }
     },
+    "libgit2": {
+      "flake": false,
+      "locked": {
+        "lastModified": 1697646580,
+        "narHash": "sha256-oX4Z3S9WtJlwvj0uH9HlYcWv+x1hqp8mhXl7HsLu2f0=",
+        "owner": "libgit2",
+        "repo": "libgit2",
+        "rev": "45fd9ed7ae1a9b74b957ef4f337bc3c8b3df01b5",
+        "type": "github"
+      },
+      "original": {
+        "owner": "libgit2",
+        "repo": "libgit2",
+        "type": "github"
+      }
+    },
     "lowdown-src": {
       "flake": false,
       "locked": {
@@ -35,6 +51,7 @@
     "nix": {
       "inputs": {
         "flake-compat": "flake-compat",
+        "libgit2": "libgit2",
         "lowdown-src": "lowdown-src",
         "nixpkgs": [
           "nixpkgs"
@@ -42,16 +59,15 @@
         "nixpkgs-regression": "nixpkgs-regression"
       },
       "locked": {
-        "lastModified": 1701122567,
-        "narHash": "sha256-iA8DqS+W2fWTfR+nNJSvMHqQ+4NpYMRT3b+2zS6JTvE=",
+        "lastModified": 1701948782,
+        "narHash": "sha256-rEu4hLHZIy3fgf88BpiaVfl79hNSESRfQNbmxAO5uzg=",
         "owner": "NixOS",
         "repo": "nix",
-        "rev": "50f8f1c8bc019a4c0fd098b9ac674b94cfc6af0d",
+        "rev": "c3827ff6348a4d5199eaddf8dbc2ca2e2ef46ec5",
         "type": "github"
       },
       "original": {
         "owner": "NixOS",
-        "ref": "2.19.2",
         "repo": "nix",
         "type": "github"
       }