nixpkgs: make allowUnfree default to true (used by qweered/hyprnixos via nixpkgs-patcher)

allow-unfree-by-default.patch

Make nixpkgs' `allowUnfree` default to true.

This is what github:numtide/nixpkgs-unfree does, minus the second nixpkgs
instance: that flake is `import nixpkgs { config.allowUnfree = true; ... }`
re-exposed as `legacyPackages`, so it carries no nixpkgs source tree of its
own -- nixpkgs-patcher and `nixpkgs.flake.source` both need a real tree, and
its GitHub tarball pin would not share a store path with our channel tarball.

`nixpkgs.config.allowUnfree` already covers the system closure. What this
patch adds is the ad-hoc CLI: `nixpkgs.flake.source` pins the registry and
NIX_PATH to the patched tree, and nixpkgs' own `legacyPackages` is
`import ./. { }` with no config, so flipping the option default is what makes
`nix shell nixpkgs#<unfree>` work purely, without NIXPKGS_ALLOW_UNFREE=1.

`pkgs/top-level/config.nix` is eval-only -- it is never copied into a
derivation -- so no package rebuilds. The only thing that changes is the
patched-nixpkgs tree itself and the registry/NIX_PATH pin that follows it.

diff --git a/pkgs/top-level/config.nix b/pkgs/top-level/config.nix
--- a/pkgs/top-level/config.nix
+++ b/pkgs/top-level/config.nix
@@ -251,7 +251,7 @@
 
     allowUnfree = mkOption {
       type = types.bool;
-      default = false;
+      default = true;
       # getEnv part is in check-meta.nix
       defaultText = literalExpression ''false || builtins.getEnv "NIXPKGS_ALLOW_UNFREE" == "1"'';
       description = ''