nix: revert default to 2.18 for now (#343032)

nixos/modules/installer/tools/nix-fallback-paths.nix

@@ -1,8 +1,7 @@
 {
-  x86_64-linux = "/nix/store/fmfy9zigxns8f1wfb4v2arf1jmfdjpjc-nix-2.24.6";
-  i686-linux = "/nix/store/bl95c09pcihf2fdqpzjyjk4bdq0bsizm-nix-2.24.6";
-  aarch64-linux = "/nix/store/2sbzgmvas19iq3nhg8xbnd8k0khahk34-nix-2.24.6";
-  riscv64-linux = "/nix/store/7yy1x9sx83wm77mjawd953d6a6wb669q-nix-riscv64-unknown-linux-gnu-2.24.6";
-  x86_64-darwin = "/nix/store/vs6bbxkwxqr828q8rj1xlbmsbnx1ry6z-nix-2.24.6";
-  aarch64-darwin = "/nix/store/d88r5b1qv1fvz2j9qndz8sr31mqgz45x-nix-2.24.6";
+  x86_64-linux = "/nix/store/f409bhlpp0xkzvdz95qr2yvfjfi8r9jc-nix-2.18.5";
+  i686-linux = "/nix/store/ra39jzrxq3bcpf55aahwv5037akvylf5-nix-2.18.5";
+  aarch64-linux = "/nix/store/xiw8a4jbnw18svgdb04hyqzg5bsjspqf-nix-2.18.5";
+  x86_64-darwin = "/nix/store/k2gzx7i90x3h2c8g6xdi1jkwbl6ic895-nix-2.18.5";
+  aarch64-darwin = "/nix/store/rqwymbndaqxma6p8s5brcl9k32n5xx54-nix-2.18.5";
 }

pkgs/development/libraries/nix-plugins/default.nix

@@ -2,21 +2,18 @@
 
 stdenv.mkDerivation rec {
   pname = "nix-plugins";
-  version = "15.0.0";
+  version = "14.0.0";
 
   src = fetchFromGitHub {
     owner = "shlevy";
     repo = "nix-plugins";
     rev = version;
-    hash = "sha256-C4VqKHi6nVAHuXVhqvTRRyn0Bb619ez4LzgUWPH1cbM=";
+    hash = "sha256-RDKAuLwcZ3Pbn5JUDmGBcfD0xbM6Jud2ouXh/YKpfS8=";
   };
 
   nativeBuildInputs = [ cmake pkg-config ];
 
-  buildInputs = [
-    nix
-    boost
-  ];
+  buildInputs = [ nix boost ];
 
   meta = {
     description = "Collection of miscellaneous plugins for the nix expression language";

pkgs/tools/package-management/nix/default.nix

@@ -241,7 +241,7 @@ in lib.makeExtensible (self: ({
     else
       nix;
 
-  stable = addFallbackPathsCheck self.nix_2_24;
+  stable = addFallbackPathsCheck self.nix_2_18;
 } // lib.optionalAttrs config.allowAliases (
   lib.listToAttrs (map (
     minor: