ghc-toolchain: Build should set `-package-env=-`

Summary

I had cabal install --lib some libraries globally, which through the package-env mechanism caused the build of ghc-toolchain to fail in the following way:

utils/ghc-toolchain/src/GHC/Toolchain/Monad.hs:32:1: error:
    Could not load module `Control.Monad.Trans.Class'
    It is a member of the hidden package `transformers-0.6.1.0'.
    You can run `:set -package transformers' to expose it.
    (Note: this unloads all the modules in the current scope.)
    It is a member of the hidden package `transformers-0.5.6.2'.
    You can run `:set -package transformers' to expose it.
    (Note: this unloads all the modules in the current scope.)
    Use -v (or `:set -v` in ghci) to see a list of the files searched for.
   |
32 | import Control.Monad.Trans.Class
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

...

I resolved this via the following diff:

diff --git a/m4/ghc_toolchain.m4 b/m4/ghc_toolchain.m4
index 128b285aee..6f3fa1c703 100644
--- a/m4/ghc_toolchain.m4
+++ b/m4/ghc_toolchain.m4
@@ -148,6 +148,7 @@ AC_DEFUN([FIND_GHC_TOOLCHAIN_BIN],[
                 -ilibraries/ghc-platform/src -iutils/ghc-toolchain/src \
                 -XNoImplicitPrelude \
                 -odir actmp-ghc-toolchain -hidir actmp-ghc-toolchain \
+                -package-env=- \
                 utils/ghc-toolchain/exe/Main.hs -o acghc-toolchain || AC_MSG_ERROR([Could not compile ghc-toolchain])
             GHC_TOOLCHAIN_BIN="./acghc-toolchain"
             ;;

Which I will upstream momentarily.