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.

---------

Co-authored-by: Zanie Blue <contact@zanie.dev>

crates/uv-python/src/installation.rs

@@ -135,7 +135,7 @@ impl PythonInstallation {
         python_install_mirror: Option<&str>,
         pypy_install_mirror: Option<&str>,
     ) -> Result<Self, Error> {
-        let installations = ManagedPythonInstallations::from_settings()?.init()?;
+        let installations = ManagedPythonInstallations::from_settings(None)?.init()?;
         let installations_dir = installations.root();
         let scratch_dir = installations.scratch();
         let _lock = installations.lock().await?;