flake: switch hydra nixpkgs follow to stable channel

flake.lock

@@ -139,7 +139,7 @@
       "inputs": {
         "nix": "nix",
         "nixpkgs": [
-          "nixpkgs"
+          "nixpkgs-stable"
         ]
       },
       "locked": {
@@ -300,6 +300,22 @@
         "type": "github"
       }
     },
+    "nixpkgs-stable": {
+      "locked": {
+        "lastModified": 1664491521,
+        "narHash": "sha256-1Zj7Vi7zswETSHXueX8Oh1OA2f68DS7TOHoelLukA1U=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "c9389643ae662834fd584ffa1898dc52956e1fba",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixos-22.05-small",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
     "nixpkgs-update": {
       "inputs": {
         "flake-compat": "flake-compat",
@@ -446,6 +462,7 @@
         "hercules-ci-effects": "hercules-ci-effects",
         "hydra": "hydra",
         "nixpkgs": "nixpkgs_4",
+        "nixpkgs-stable": "nixpkgs-stable",
         "nixpkgs-update": "nixpkgs-update",
         "nixpkgs-update-github-releases": "nixpkgs-update-github-releases",
         "nixpkgs-update-pypi-releases": "nixpkgs-update-pypi-releases",

flake.nix

@@ -12,6 +12,7 @@
 
   inputs = {
     nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable-small";
+    nixpkgs-stable.url = "github:NixOS/nixpkgs/nixos-22.05-small";
     nixpkgs-update.url = "github:ryantm/nixpkgs-update";
     nixpkgs-update-github-releases.url = "github:ryantm/nixpkgs-update-github-releases";
     nixpkgs-update-github-releases.flake = false;
@@ -22,7 +23,9 @@
     hercules-ci-effects.url = "github:hercules-ci/hercules-ci-effects";
     hercules-ci-agent.url = "github:hercules-ci/hercules-ci-agent/master";
     hydra.url = "github:NixOS/hydra";
-    hydra.inputs.nixpkgs.follows = "nixpkgs";
+    # switch back to unstable when NixOS/nix moves to 22.11
+    #hydra.inputs.nixpkgs.follows = "nixpkgs";
+    hydra.inputs.nixpkgs.follows = "nixpkgs-stable";
 
     flake-parts.url = "github:hercules-ci/flake-parts";
     flake-parts.inputs.nixpkgs.follows = "nixpkgs";