splice.nix: make `pkgs` `splicedPackages` when required

This will make `pkgs` used in `callPackage`, and `pkgsCross.X.pkgs` have
packages with `__spliced`.

https://www.github.com/NixOS/nixpkgs/blob/3029741718f4c765fbc5ebf76bea3d6c8ff15fe5/pkgs/development/interpreters/python/passthrufun.nix#L37

https://www.github.com/NixOS/nixpkgs/blob/d2bd9a39dec88eddd5c192abee69939e67f43d12/pkgs/top-level/python-packages.nix#L10720

```
nix-repl>
pkgsCross.aarch64-multiplatform.python3Packages.protobuf4.protobuf.__spliced
error:
       … while evaluating the attribute
'aarch64-multiplatform.python3Packages.protobuf4.protobuf.__spliced'
         at
/home/artturin/nixgits/my-nixpkgs/.worktree/1/pkgs/development/python-modules/protobuf/4.nix:119:13:
          118|   passthru = {
          119|     inherit protobuf;
             |             ^
          120|   };

       error: attribute '__spliced' missing
       at «string»:1:1:
            1|
pkgsCross.aarch64-multiplatform.python3Packages.protobuf4.protobuf.__spliced
             | ^
```

to

```
nix-repl>
pkgsCross.aarch64-multiplatform.python3Packages.protobuf4.protobuf.__spliced
{
  buildBuild = «derivation
/nix/store/s7da5mfvx4h1n86j78knaj9cprglxqz6-protobuf-25.4.drv»;
  buildHost = «derivation
/nix/store/s7da5mfvx4h1n86j78knaj9cprglxqz6-protobuf-25.4.drv»;
  buildTarget = «repeated»;
  hostHost = «derivation
/nix/store/mszvybzs4zxh43awyrjnybsfcb265n9r-protobuf-aarch64-unknown-linux-gnu-25.4.drv»;
  hostTarget = «repeated»;
}
```

Co-authored-by: Wolfgang Walther <walther@technowledgy.de>

0 files changed, 0 insertions, 0 deletions