Fix EM_CACHE issue

Author: hamishmack

builder/comp-builder.nix

@@ -447,6 +447,10 @@ let
       (lib.optionalString stdenv.hostPlatform.isGhcjs ''
       export HOME=$(mktemp -d)
       export EM_CACHE=$(mktemp -d)
+      if [ -d ${pkgsBuildBuild.emscripten}/share/emscripten/cache ]; then
+        cp -r ${pkgsBuildBuild.emscripten}/share/emscripten/cache/* $EM_CACHE/
+        chmod +w -R $EM_CACHE
+      fi
       '') +
       (lib.optionalString (!canCleanSource) ''
       echo "Cleaning component source not supported, leaving it un-cleaned"

compiler/ghc/default.nix

@@ -435,6 +435,10 @@ haskell-nix.haskellLib.makeCompilerDeps (stdenv.mkDerivation (rec {
         ''
     ) + ''
         export EM_CACHE=$(mktemp -d)
+        if [ -d ${targetCC}/share/emscripten/cache ]; then
+          cp -r ${targetCC}/share/emscripten/cache/* $EM_CACHE/
+          chmod +w -R $EM_CACHE
+        fi
         mv config.sub.ghcjs config.sub
     '')
     # GHC is a bit confused on its cross terminology, as these would normally be