mkKdeDerivation: do not use stdenv from qt6Packages

Previously, mkKdeDerivation used stdenv from `qt6Packages`, but now it
is deprecated and raises an eval warning. This fix removes `stdenv` from
`qt6Packages` before calling `mkKdeDerivation`.

I also created similar PR NixOS#371444, but it exposed `kdePackages.stdenv`
which is not good. And this commit is not a breaking change, because
`kdePackages.stdenv` was provided for only a few hours in master.

Author: PerchunPak

pkgs/kde/default.nix

@@ -41,7 +41,10 @@ let
       ) allUrls;
     in
     (
-      qt6Packages
+      # This removes reference to deprecated `qt6Packages.stdenv`
+      # so when a KDE package asks for a stdenv, it won't trigger
+      # an eval warning
+      (lib.removeAttrs qt6Packages [ "stdenv" ])
       // frameworks
       // gear
       // plasma
@@ -66,10 +69,6 @@ let
         # Alias because it's just data
         plasma-wayland-protocols = libsForQt5.plasma-wayland-protocols;
 
-        # Alias because `self.callPackage` would give deprecated
-        # `qt6Packages.stdenv` when asked for `stdenv`
-        inherit stdenv;
-
         selenium-webdriver-at-spi = null; # Used for integration tests that we don't run, stub
 
         alpaka = self.callPackage ./misc/alpaka { };

pkgs/top-level/qt6-packages.nix

@@ -126,5 +126,6 @@ makeScopeWithSplicing' {
   xwaylandvideobridge = kdePackages.callPackage ../tools/wayland/xwaylandvideobridge { };
   });
 } // lib.optionalAttrs config.allowAliases {
+  # when removing, don't forget to remove a workaround in `pkgs/kde/default.nix`
   stdenv = lib.warn "qt6Packages.stdenv is deprecated. Use stdenv instead." stdenv; # Added for 25.05
 }