List 'setEnv' as opposite of 'getEnv'

It seems best to direct users to use 'System.Environment.setEnv'
rather than 'System.Posix.Env.putEnv'. This is due to 'setEnv' being
located in the same module as 'getEnv' and my virtue of working on
Windows platform, whereas 'putEnv' does not have that quality because
it's part of the 'unix' package.

(cherry picked from commit c74fcf5d27744c67b09b29a78029c31c9371cf41)

libraries/base/System/Environment.hs

@@ -123,8 +123,8 @@ basename f = go f f
 
 
 -- | Computation 'getEnv' @var@ returns the value
--- of the environment variable @var@. For the inverse, POSIX users
--- can use 'System.Posix.Env.putEnv'.
+-- of the environment variable @var@. For the inverse, the
+-- `System.Environment.setEnv` function can be used.
 --
 -- This computation may fail with:
 --