Add support for `HF_TOKEN` (#14797)
## Summary
If `HF_TOKEN` is set, we'll automatically wire it up to authenticate
requests when hitting private `huggingface.co` URLs in `uv run`.
## Test Plan
An unauthenticated request:
```
> cargo run -- run https://huggingface.co/datasets/cmarsh/test/resolve/main/main.py
File "/var/folders/nt/6gf2v7_s3k13zq_t3944rwz40000gn/T/mainYadr5M.py", line 1
Invalid username or password.
^^^^^^^^
SyntaxError: invalid syntax
```
An authenticated request:
```
> HF_TOKEN=hf_... cargo run run https://huggingface.co/datasets/cmarsh/test/resolve/main/main.py
Hello from main.py!
```
