misc/ios: fix an error when getenv encounters unset variable

The error message should indicate the name of the unset variable,
rather than the value. The value will alwayse be empty.

Change-Id: I6f6c165074dfce857b6523703a890d205423cd28
Reviewed-on: https://go-review.googlesource.com/16555


Reviewed-by: David Crawshaw <crawshaw@golang.org>