ghc-toolchain: build with `-package-env=-` (#24131)

Otherwise globally installed libraries (via `cabal install --lib`)
break the build.

Fixes #24131.

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"
             ;;