Make Python and PyPy install mirrors configurable in uv.toml (#8695)

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## Summary

Adds python-install-mirror and pypy-install-mirror as keys for uv.toml,
and cli args for `uv python install`.

Could leave the cli args out if we think the env vars and configs are
sufficient.

Fixes #8186 

<!-- What's the purpose of the change? What does it do, and why? -->

---------

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

crates/uv-cli/src/lib.rs

@@ -3938,6 +3938,22 @@ pub struct PythonInstallArgs {
     /// See `uv help python` to view supported request formats.
     pub targets: Vec<String>,
 
+    /// Set the URL to use as the source for downloading Python installations.
+    ///
+    /// The provided URL will replace `https://github.com/indygreg/python-build-standalone/releases/download` in, e.g., `https://github.com/indygreg/python-build-standalone/releases/download/20240713/cpython-3.12.4%2B20240713-aarch64-apple-darwin-install_only.tar.gz`.
+    ///
+    /// Distributions can be read from a local directory by using the `file://` URL scheme.
+    #[arg(long, env = EnvVars::UV_PYTHON_INSTALL_MIRROR)]
+    pub mirror: Option<String>,
+
+    /// Set the URL to use as the source for downloading PyPy installations.
+    ///
+    /// The provided URL will replace `https://downloads.python.org/pypy` in, e.g., `https://downloads.python.org/pypy/pypy3.8-v7.3.7-osx64.tar.bz2`.
+    ///
+    /// Distributions can be read from a local directory by using the `file://` URL scheme.
+    #[arg(long, env = EnvVars::UV_PYPY_INSTALL_MIRROR)]
+    pub pypy_mirror: Option<String>,
+
     /// Reinstall the requested Python version, if it's already installed.
     ///
     /// By default, uv will exit successfully if the version is already