Backwards breaking change for setEnv

[Edward Z. Yang]

Hello all,

I wanted to bring this bug to the attention of the CLC

    https://ghc.haskell.org/trac/ghc/ticket/12494#ticket

I propose that we change 'setEnv k ""' to set the environment
variable to an empty string, rather than delete the environment
variable.  The existing behavior was motivated by a Windows
"quirk" that actually doesn't exist (details in ticket.)

Thanks,
Edward