Update to Nix 2.19

Flake lock file updates:

• Updated input 'nix':
    'github:NixOS/nix/f5f4de6a550327b4b1a06123c2e450f1b92c73b6' (2023-10-02)
  → 'github:NixOS/nix/50f8f1c8bc019a4c0fd098b9ac674b94cfc6af0d' (2023-11-27)

src/hydra-queue-runner/nar-extractor.hh

@@ -1,13 +1,13 @@
 #pragma once
 
-#include "fs-accessor.hh"
+#include "source-accessor.hh"
 #include "types.hh"
 #include "serialise.hh"
 #include "hash.hh"
 
 struct NarMemberData
 {
-    nix::FSAccessor::Type type;
+    nix::SourceAccessor::Type type;
     std::optional<uint64_t> fileSize;
     std::optional<std::string> contents;
     std::optional<nix::Hash> sha256;