There are multiple ways to achieve this:
1) The env command being discussed is actually "stack exec env". Though it includes the full environment rather than stack exclusive. You can use "stack path" to print the stack exclusive environment. You can also use "stack path --<flag>" to pick specific items from that env.
2) Using "stack exec bash" is a very convenient way as suggested by Christopher Allen.
3) But I prefer to just use "export PATH=$(stack path --bin-path)" instead which only sets the PATH. The full environment (when using env or bash) also includes GHC_PACKAGE_PATH which cabal does not like. So if you want to use cabal on stack installed ghc just setting PATH works fine.
I think stack has a lot of flexibility and features, in fact too many. Usually there is already a way to achieve something that you want. Though there are areas where the user experience can be made better including cosmetic stuff like not throwing confusing or unnecessary warnings.
-harendra