Add warning when target version does not match build version (#1072)
Follow-up to https://github.com/astral-sh/puffin/pull/1040 adding a
user-facing warning when we cannot build with their requested version.
e.g.
```
❯ cargo run -- pip compile requirements.in --python-version 3.11.4 --no-build
Resolved 8 packages in 483ms
❯ cargo run -- pip compile requirements.in --python-version 3.11.4
warning: The requested Python version 3.11.4 is not available; 3.11.7 will be used to build dependencies instead.
Resolved 8 packages in 71ms
❯ cargo run -- pip compile requirements.in --python-version 3.11
Resolved 8 packages in 71ms
```
