Two build fixes found while getting the build working on FreeBSD.
1. **Remove hardcoded `#!/bin/bash` paths and use /usr/bin/env instead
/**
2. **env_remove("YARN_BINARY")**
Tested on FreeBSD 16.0-CURRENT amd64.
A second series adds FreeBSD support with `platform` variants,
environment overrides, PyQt6 handling and docs.
Closes#5301
2d88e386dc