only target single nix version in repo

.nix-version

@@ -0,0 +1 @@
+unstable

flake.nix

@@ -4,53 +4,43 @@
   inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
   inputs.flake-utils.url = "github:numtide/flake-utils";
 
-  outputs = { self, nixpkgs, flake-utils }:
-    flake-utils.lib.eachDefaultSystem (system:
+  outputs =
+    { self
+    , nixpkgs
+    , flake-utils
+    }:
+    flake-utils.lib.eachDefaultSystem (
+      system:
       let
+        nixVersion = nixpkgs.lib.fileContents ./.nix-version;
         pkgs = nixpkgs.legacyPackages.${system};
         inherit (pkgs) stdenv;
-        drvArgs = { srcDir = self; };
+        devShell = self.devShells.${system}.default;
+        drvArgs = {
+          srcDir = self;
+          nix = if nixVersion == "unstable" then pkgs.nixUnstable else pkgs.nixVersions."nix_${nixVersion}";
+        };
       in
-      rec {
+      {
         packages.nix-eval-jobs = pkgs.callPackage ./default.nix drvArgs;
 
-        checks =
-          let
-            mkVariant = nix: (packages.nix-eval-jobs.override {
-              inherit nix;
-            }).overrideAttrs (_: {
-              name = "nix-eval-jobs-${nix.version}";
-              inherit (nix) version;
-            });
-          in
-          {
+        checks.treefmt = stdenv.mkDerivation {
+          name = "treefmt-check";
+          src = self;
+          nativeBuildInputs = devShell.nativeBuildInputs;
+          dontConfigure = true;
 
-            treefmt =
-              let
-                devShell = devShells.default;
-              in
-              stdenv.mkDerivation {
-                name = "treefmt-check";
-                src = self;
-                nativeBuildInputs = devShell.nativeBuildInputs;
-                dontConfigure = true;
+          inherit (devShell) NODE_PATH;
 
-                inherit (devShell) NODE_PATH;
+          buildPhase = ''
+            env HOME=$(mktemp -d) treefmt --fail-on-change
+          '';
 
-                buildPhase = ''
-                  env HOME=$(mktemp -d) treefmt --fail-on-change
-                '';
-
-                installPhase = "touch $out";
-              };
-
-            build = mkVariant pkgs.nix;
-            build-unstable = mkVariant pkgs.nixUnstable;
-          };
+          installPhase = "touch $out";
+        };
 
         packages.default = self.packages.${system}.nix-eval-jobs;
         devShells.default = pkgs.callPackage ./shell.nix drvArgs;
-
       }
     );
 }

shell.nix

@@ -11,15 +11,15 @@
     import nixpkgs { }
   )
 , srcDir ? null
+, nix
 }:
 
 let
   inherit (pkgs) lib stdenv;
-  nix = pkgs.nixUnstable;
 
 in
 (pkgs.callPackage ./default.nix {
-  inherit nix srcDir;
+  inherit srcDir nix;
 }).overrideAttrs (old: {
 
   src = null;