fix: TUI spawn: reset BUN_OPTIONS

[veracioux]

I often do something like export BUN_OPTIONS='--inspect-wait=localhost:6499/' before running bun dev.
In the opentui branch, I usually use bun dev spawn so I can get working debugger endpoints in server.ts. Before this fix, the attach subprocess spawned by bun dev spawn would inherit BUN_OPTIONS, causing a port conflict.

Even as a more general principle, I think we shouldn't blindly pass BUN_OPTIONS to subprocesses.