edk2: Fix cross-compile

deep-merge env in mkDerivation to preserve GCC5 prefix

  The shallow `//` merge in edk2.mkDerivation allowed consumers' `env`
  attrs to completely overwrite the base `env` containing GCC5_*_PREFIX,
  causing cross-compilation failures (e.g. x86_64 -> aarch64 OVMF) with
  `gcc: error: unrecognized command-line option '-mlittle-endian'`.

  Exclude `env` from the consumer attrs spread and explicitly deep-merge
  it, ensuring GCC5_*_PREFIX is always set as a base while consumer `env`
  keys (NIX_CFLAGS_COMPILE, PYTHON_COMMAND, etc.) are merged on top.

  Fixes regression from c7b9734f4a ("edk2: move env variable(s) into
  env for structuredAttrs").

Signed-off-by: Brian McGillion <bmg.avoin@gmail.com>

pkgs/by-name/ed/edk2/package.nix

@@ -182,8 +182,6 @@ stdenv.mkDerivation (finalAttrs: {
           ++ attrs.nativeBuildInputs or [ ];
           strictDeps = true;
 
-          env.${"GCC5_${targetArch}_PREFIX"} = stdenv.cc.targetPrefix;
-
           prePatch = ''
             rm -rf BaseTools
             ln -sv ${buildPackages.edk2}/BaseTools BaseTools
@@ -211,7 +209,14 @@ stdenv.mkDerivation (finalAttrs: {
         // removeAttrs attrs [
           "nativeBuildInputs"
           "depsBuildBuild"
+          "env"
         ]
+        // {
+          env = {
+            ${"GCC5_${targetArch}_PREFIX"} = stdenv.cc.targetPrefix;
+          }
+          // (attrs.env or { });
+        }
       );
   };
 })