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>

docs/reference/cli.md

@@ -4553,6 +4553,13 @@ uv python install [OPTIONS] [TARGETS]...
 
 </dd><dt><code>--help</code>, <code>-h</code></dt><dd><p>Display the concise help for this command</p>
 
+</dd><dt><code>--install-dir</code>, <code>-i</code> <i>install-dir</i></dt><dd><p>The directory to store the Python installation in.</p>
+
+<p>If provided, <code>UV_PYTHON_INSTALL_DIR</code> will need to be set for subsequent operations for uv to discover the Python installation.</p>
+
+<p>See <code>uv python dir</code> to view the current Python installation directory. Defaults to <code>~/.local/share/uv/python</code>.</p>
+
+<p>May also be set with the <code>UV_PYTHON_INSTALL_DIR</code> environment variable.</p>
 </dd><dt><code>--mirror</code> <i>mirror</i></dt><dd><p>Set the URL to use as the source for downloading Python installations.</p>
 
 <p>The provided URL will replace <code>https://github.com/indygreg/python-build-standalone/releases/download</code> in, e.g., <code>https://github.com/indygreg/python-build-standalone/releases/download/20240713/cpython-3.12.4%2B20240713-aarch64-apple-darwin-install_only.tar.gz</code>.</p>
@@ -5114,6 +5121,9 @@ uv python uninstall [OPTIONS] <TARGETS>...
 
 </dd><dt><code>--help</code>, <code>-h</code></dt><dd><p>Display the concise help for this command</p>
 
+</dd><dt><code>--install-dir</code>, <code>-i</code> <i>install-dir</i></dt><dd><p>The directory where the Python was installed</p>
+
+<p>May also be set with the <code>UV_PYTHON_INSTALL_DIR</code> environment variable.</p>
 </dd><dt><code>--native-tls</code></dt><dd><p>Whether to load TLS certificates from the platform&#8217;s native certificate store.</p>
 
 <p>By default, uv loads certificates from the bundled <code>webpki-roots</code> crate. The <code>webpki-roots</code> are a reliable set of trust roots from Mozilla, and including them in uv improves portability and performance (especially on macOS).</p>