Firefox perf tests don't run if CHROME_BIN is set

### Issue

When trying to run perf tests using `ADAPTERS=idb COUCH_HOST='http://admin:password@localhost:5984' CLIENT=selenium:firefox PERF=1 npm test`, seeing the following error:
```
The environment you requested was unavailable.
unknown error: DevToolsActivePort file doesn't exist
```

### Info
- Environment: Node.js 12
- Platform: ubuntu
- Adapter: idb
- Server: CouchDB 1.7.1

### Fix

This can be fixed by _clearing_ the `CHROME_BIN` env var.