Use the Nixpkgs fileset library to filter source

Now I can change Nix files without causing rebuilds.

flake.nix

@@ -10,7 +10,7 @@
   # 23.11 and has `lib.fileset`.
   inputs.nixpkgs-for-fileset.url = "github:NixOS/nixpkgs/nixos-23.11";
 
-  outputs = { self, nixpkgs, nix }:
+  outputs = { self, nixpkgs, nix, nixpkgs-for-fileset }:
     let
       systems = [ "x86_64-linux" "aarch64-linux" ];
       forEachSystem = nixpkgs.lib.genAttrs systems;
@@ -65,7 +65,8 @@
         };
 
         hydra = final.callPackage ./package.nix {
-          src = self;
+          inherit (nixpkgs-for-fileset.lib) fileset;
+          rawSrc = self;
         };
       };
 

package.nix

@@ -1,7 +1,8 @@
 { stdenv
 , lib
+, fileset
 
-, src
+, rawSrc
 
 , buildEnv
 
@@ -128,13 +129,28 @@ let
       ];
   };
 
-  version = "${builtins.readFile ./version.txt}.${builtins.substring 0 8 (src.lastModifiedDate or "19700101")}.${src.shortRev or "DIRTY"}";
+  version = "${builtins.readFile ./version.txt}.${builtins.substring 0 8 (rawSrc.lastModifiedDate or "19700101")}.${rawSrc.shortRev or "DIRTY"}";
 in
-stdenv.mkDerivation {
+stdenv.mkDerivation (finalAttrs: {
   pname = "hydra";
   inherit version;
 
-  inherit src;
+  src = fileset.toSource {
+    root = ./.;
+    fileset = fileset.unions ([
+      ./version.txt
+      ./configure.ac
+      ./Makefile.am
+      ./src
+      ./doc
+      ./hydra-module.nix
+      # TODO only when `doCheck`
+      ./t
+    ] ++ lib.optionals finalAttrs.doCheck [
+      ./.perlcriticrc
+      ./.yath.rc
+    ]);
+  };
 
   strictDeps = true;
 
@@ -251,4 +267,4 @@ stdenv.mkDerivation {
 
   meta.description = "Build of Hydra on ${stdenv.system}";
   passthru = { inherit perlDeps nix; };
-}
+})