pkgsStrict: init

enabling top-level access to this config supports the effort to enable
`strictDeps` by default:
<https://github.com/NixOS/nixpkgs/issues/178468>

PRs to support musl, static, and cross compiled builds benefit from
simple instructions the reviewer can observe improvements, e.g.:

- "fixes build for `pkgsMusl.hello`"
- "fixes `nix-build -A pkgsStatic.hello`"
- "fixes `pkgsCross.aarch64-multiplatform.hello`"

now strictDeps-related changes can benefit from this same idiom:

- "fixes build for `pkgsStrict.hello`"

pkgs/top-level/variants.nix

@@ -173,4 +173,10 @@ self: super: {
     ]
     ++ overlays;
   };
+
+  pkgsStrict = nixpkgsFun {
+    config = super.config // {
+      strictDepsByDefault = true;
+    };
+  };
 }