feat: enable env var for preflight (#7754)
### Description
Fixes https://github.com/vercel/turbo/issues/7753
- Enabled `TURBO_PREFLIGHT` env var as a config source with `1` and
`true` being considered truthy
- Pipe `--preflight` CLI arg into our config
- Use `config.pipeline()` instead of directly reading value from CLI
args
### Testing Instructions
Expanded unit tests to cover `TURBO_PREFLIGHT`
Closes TURBO-2650
