[Sandbox] Don't inject the HF token in sandbox jobs as SBX_DL_TOKEN (#4583)
The bootstrap script downloaded the sbx-server binary with the user's HF
token as a bearer token. The server bucket is public, so the header was
unnecessary - and it put a reusable HF token in the job environment even
with forward_hf_token=False, where the image's own /bin/sh could read it.
Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>