There is no browsers key in the environment schema, and unknown keys are dropped rather than rejected — so your line is parsed, discarded, and never reaches anything.
Browsers are a project-level decision. An environment describes where the app is, not how you drive it, and the only project sections an environment file may patch are screenshots, heal, timeouts and perf.
Three ways to get what you want:
bash
sdods run -p demo-shop -e local -l ui -b chromium
a process with browsers: [chromium] for the recipe you repeat, or SDODS_* in a shell profile if it is really per-machine. The flag is layer 6 and wins over everything.