test/ENV_spec.rb: Fix prepend_path for Linuxbrew
test/ENV_spec.rb: Fix prepend_path for Linuxbrew

This test fails on CircleCI on Ubuntu 14, because
/usr/libexec does not exist.

rwhogg/brew,Linuxbrew/brew,maxim-belkin/brew,Linuxbrew/brew,hanxue/linuxbrew,Linuxbrew/brew,Linuxbrew/brew,rwhogg/brew,maxim-belkin/brew,hanxue/linuxbrew,maxim-belkin/brew,hanxue/linuxbrew,rwhogg/brew