Revert "busybox: fix static builds"

This reverts commit b4f6931acd.

Broke busybox-sandbox-shell

https://hydra.nixos.org/build/100470231

pkgs/os-specific/linux/busybox/default.nix

@@ -1,7 +1,7 @@
 { stdenv, lib, buildPackages, fetchurl
 , enableStatic ? false
 , enableMinimal ? false
-, useMusl ? stdenv.hostPlatform.libc == "musl"
+, useMusl ? stdenv.hostPlatform.libc == "musl", musl
 , extraConfig ? ""
 }:
 
@@ -88,6 +88,10 @@ stdenv.mkDerivation rec {
     runHook postConfigure
   '';
 
+  postConfigure = lib.optionalString useMusl ''
+    makeFlagsArray+=("CC=${stdenv.cc.targetPrefix}cc -isystem ${musl.dev}/include -B${musl}/lib -L${musl}/lib")
+  '';
+
   depsBuildBuild = [ buildPackages.stdenv.cc ];
 
   buildInputs = lib.optionals (enableStatic && !useMusl && stdenv.cc.libc ? static) [ stdenv.cc.libc stdenv.cc.libc.static ];