fix: pass src from flake to package.nix

Relative paths in package.nix don't resolve correctly when
the flake is used as an input in another flake. Pass src
explicitly from the flake instead.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

flake.nix

@@ -17,7 +17,7 @@
 
       # Overlay to add systant to pkgs
       overlays.default = final: prev: {
-        systant = final.callPackage ./nix/package.nix { };
+        systant = final.callPackage ./nix/package.nix { src = self; };
       };
     }
     //

nix/package.nix

@@ -4,6 +4,7 @@
   stdenv,
   bun,
   cacert,
+  src,  # passed from flake.nix
 }:
 
 let
@@ -14,15 +15,7 @@ let
     pname = "systant-deps";
     version = "0.1.0";
 
-    src = lib.fileset.toSource {
-      root = ./..;
-      fileset = lib.fileset.unions [
-        ./../package.json
-        ./../bun.lock
-      ];
-    };
-
-    nativeBuildInputs = [ bun cacert ];
+    inherit src;
 
     buildPhase = ''
       export HOME=$TMPDIR
@@ -33,6 +26,8 @@ let
       cp -r node_modules $out
     '';
 
+    nativeBuildInputs = [ bun cacert ];
+
     outputHashMode = "recursive";
     outputHashAlgo = "sha256";
     # To update: nix build .#systant 2>&1 | grep 'got:'
@@ -43,15 +38,7 @@ stdenv.mkDerivation {
   pname = "systant";
   version = "0.1.0";
 
-  src = lib.fileset.toSource {
-    root = ./..;
-    fileset = lib.fileset.unions [
-      ./../index.ts
-      ./../src
-      ./../package.json
-      ./../tsconfig.json
-    ];
-  };
+  inherit src;
 
   nativeBuildInputs = [ bun ];