Respect `PAGER` env var when paging in `uv help` command (#5511)
## Summary
Closes https://github.com/astral-sh/uv/issues/4931.
## Test Plan
Tried running the following commands locally to make sure that all cases
work:
```
unset PAGER
cargo run -- help venv
```
With no pager set, `uv` correctly finds `less` on the system as it did
before and passes the help output to it.
---
```
PAGER= cargo run -- help venv
```
This correctly prints out to stdout and does not use any pager.
---
```
PAGER=most cargo run -- help venv
```
This correctly opens the `most` pager as shown below:
<img width="1917" alt="Screenshot 2024-07-27 at 5 14 42 PM"
src="https://github.com/user-attachments/assets/dfaa5a83-b47e-4f5c-9be1-b0b1e9818932">
---------
Co-authored-by: Zanie Blue <contact@zanie.dev>
