`env-setup` fix for zsh <= 4.3.10

This fixes issue #5026. It used to work. The regression was
introduced in 2b3381de7c6e0924b29123c33f0627a12418f2c8.
Merge pull request #5163 from JensRantil/fix-issue-5026

`env-setup` fix for zsh <= 4.3.10