Add `--install-dir` arg to `uv python install` and `uninstall` (#7920)
## Summary
This PR adds `--install-dir` argument for the following commands:
- `uv python install`
- `uv python uninstall`
The `UV_PYTHON_INSTALL_DIR` env variable can be used to set it
(previously it was also used internally).
Any more commands we would want to add this to?
## Test Plan
For now just manual test (works on my machine hehe)
```
❯ ./target/debug/uv python install --install-dir /tmp/pythons 3.8.12
Searching for Python versions matching: Python 3.8.12
Installed Python 3.8.12 in 4.31s
+ cpython-3.8.12-linux-x86_64-gnu
❯ /tmp/pythons/cpython-3.8.12-linux-x86_64-gnu/bin/python --help
usage: /tmp/pythons/cpython-3.8.12-linux-x86_64-gnu/bin/python [option] ... [-c cmd | -m mod | file | -] [arg] ...
```
Open to add some tests after the initial feedback.
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Co-authored-by: Zanie Blue <contact@zanie.dev>