Make build-and-upload and kernels work without applicable build set (#344)

We usually want the `build-and-upload` and `kernels` commands to work,
even when there are no applicable build sets (e.g. when compiling a CUDA
kernel on macOS). This makes it easier to integrate these commands into
CI (which would otherwise fail).

This change makes it so that these commands work without applicable
build sets.

Author: danieldk

lib/gen-flake-outputs.nix

@@ -101,6 +101,16 @@ let
     else
       builtins.head buildSetsSorted;
   shellTorch = bestBuildSet.torch.variant;
+  # We need a package set for some outputs (e.g. kernels and build-and-upload),
+  # even when there is no applicable build set.
+  pkgs =
+    let
+      sorted = lib.sort configCompare (addSortOrder buildSets);
+    in
+    if sorted == [ ] then
+      throw "No build set is available for this system"
+    else
+      (builtins.head sorted).pkgs;
   headOrEmpty = l: if l == [ ] then [ ] else [ (builtins.head l) ];
 in
 {
@@ -188,11 +198,17 @@ in
             "repo-id"
           ] "kernels-community/${buildToml.general.name}" buildToml;
           branch = lib.attrByPath [ "general" "hub" "branch" ] "main" buildToml;
+          # `kernels upload` fails when there are no build variants to upload.
+          # However, we do not want this command to error out in that case, so
+          # only insert the upload command when there is something to upload.
+          uploadStr = lib.optionalString (applicableBuildSets != [ ]) ''
+            ${pkgs.python3.pkgs.kernels}/bin/kernels upload --repo-id ${repo_id} --branch ${branch} ${bundle}
+          '';
         in
         writeScriptBin "build-and-upload" ''
           #!/usr/bin/env bash
           set -euo pipefail
-          ${bestBuildSet.pkgs.python3.pkgs.kernels}/bin/kernels upload --repo-id ${repo_id} --branch ${branch} ${bundle}
+          ${uploadStr}
         '';
 
       ci =
@@ -214,7 +230,7 @@ in
         };
 
       kernels =
-        bestBuildSet.pkgs.python3.withPackages (
+        pkgs.python3.withPackages (
           ps: with ps; [
             kernel-abi-check
             kernels