Add a system-wide flake registry /etc/nix/registry.json

One application for this is pinning the 'nixpkgs' flake to the exact
revision used to build the NixOS system, e.g.

  {
      "flakes": [
          {
              "from": {
                  "id": "nixpkgs",
                  "type": "indirect"
              },
              "to": {
                  "owner": "NixOS",
                  "repo": "nixpkgs",
                  "type": "github",
                  "rev": "b0c285807d6a9f1b7562ec417c24fa1a30ecc31a"
              }
          }
      ],
      "version": 2
  }

1 files changed, 2 insertions, 1 deletions

diff --git a/src/libfetchers/registry.hh b/src/libfetchers/registry.hh
index d2eb7749b..5e8857850 100644
--- a/src/libfetchers/registry.hh
+++ b/src/libfetchers/registry.hh
@@ -12,7 +12,8 @@ struct Registry
     enum RegistryType {
         Flag = 0,
         User = 1,
-        Global = 2,
+        System = 2,
+        Global = 3,
     };
 
     RegistryType type;