Remove `Option<bool>` for `--no-cache` (#3129)
## Summary
This was unintended. We ended up reverting `Option<bool>` everywhere,
but I missed this once since it's in a separate file.
(If you use `Option<bool>`, Clap requires a value, like `--no-cache
true`.)
## Test Plan
`cargo run pip install flask --no-cache`
