Run

FOO=shell clojure -m figwheel.main -bo dev

There is a .lein-env file which also sets the variable FOO.

This is to trigger this warning

Check less target/public/cljs-out/dev/chromex/defaults.js

The warning from environ will have been emitted to the JS file:

// Compiled by ClojureScript 1.10.773 {}
Warning: environ value test for key :foo has been overwritten with original