Generate empty bundle when there is no matching framework

Before this change building `.#ci` when there is no matching backend
(e.g. when building a CUDA kernel on macOS) would fail with a Nix
evaluation error.

Change this to return an empty bundle in such cases.

Author: danieldk

flake.nix

@@ -43,10 +43,7 @@
       mkBuildPerSystem =
         buildSetPerSystem:
         builtins.mapAttrs (
-          system: buildSet:
-          import lib/build.nix {
-            inherit (nixpkgs) lib;
-          }
+          system: buildSet: nixpkgs.legacyPackages.${system}.callPackage lib/build.nix { }
         ) buildSetPerSystem;
 
       defaultBuildPerSystem = mkBuildPerSystem defaultBuildSetsPerSystem;
@@ -113,9 +110,7 @@
       system:
       let
         # Plain nixkpgs that we use to access utility funtions.
-        pkgs = import nixpkgs {
-          inherit system;
-        };
+        pkgs = nixpkgs.legacyPackages.${system};
         inherit (nixpkgs) lib;
 
         buildName = (import ./lib/build-variants.nix { inherit lib; }).buildName;

lib/build.nix

@@ -1,5 +1,6 @@
 {
   lib,
+  pkgs,
 
 # Every `buildSets` argument is a list of build sets. Each build set is
 # a attrset of the form
@@ -195,7 +196,6 @@ rec {
     }:
     let
       # We just need to get any nixpkgs for use by the path join.
-      pkgs = (builtins.head buildSets).pkgs;
       extensions = mkDistTorchExtensions {
         inherit
           buildSets

lib/join-paths/default.nix

@@ -22,5 +22,10 @@ let
     mkdir -p ${placeholder "out"}/${path}
     cp -r ${pkg}/* ${placeholder "out"}/${path}
   '';
+  prelude = ''
+    mkdir -p ${placeholder "out"}
+  '';
 in
-pkgs.runCommand name args_ (lib.concatStringsSep "\n" (lib.mapAttrsToList copyPath namePaths))
+pkgs.runCommand name args_ (
+  prelude + lib.concatStringsSep "\n" (lib.mapAttrsToList copyPath namePaths)
+)