I use "export PATH=`stack path --bin-path`" to make the stack installed ghc available in the PATH before building ghc. And that's all.
Setting the PATH works better because we do not get any extra env variables set by stack in the environment and we do not go through the stack wrapper, so it may be a little bit faster as well. The GHC_PACKAGE_PATH variable set by the stack command is especially troublesome in some cases. You can try "stack exec env" to check all vars that stack puts in your environment.
-harendra