Replace toolchain fetch with toolchain install (#4228)

## Summary

Something that looks like it was forgotten to replace in #4164.

## Test Plan

Run `cargo run toolchain install` should display the warning: ``warning:
`uv toolchain install` is experimental and may change without warning.``

Author: SigureMo

crates/uv/src/cli.rs

@@ -2014,7 +2014,7 @@ pub(crate) struct ToolchainListArgs {
 #[derive(Args)]
 #[allow(clippy::struct_excessive_bools)]
 pub(crate) struct ToolchainInstallArgs {
-    /// The toolchain to fetch.
+    /// The toolchain to install.
     ///
     /// If not provided, the latest available version will be installed.
     pub(crate) target: Option<String>,

crates/uv/src/commands/toolchain/install.rs

@@ -23,7 +23,7 @@ pub(crate) async fn install(
     printer: Printer,
 ) -> Result<ExitStatus> {
     if preview.is_disabled() {
-        warn_user!("`uv toolchain fetch` is experimental and may change without warning.");
+        warn_user!("`uv toolchain install` is experimental and may change without warning.");
     }
 
     let toolchains = InstalledToolchains::from_settings()?.init()?;

crates/uv/src/settings.rs

@@ -266,7 +266,7 @@ impl ToolchainListSettings {
     }
 }
 
-/// The resolved settings to use for a `toolchain fetch` invocation.
+/// The resolved settings to use for a `toolchain install` invocation.
 #[allow(clippy::struct_excessive_bools)]
 #[derive(Debug, Clone)]
 pub(crate) struct ToolchainInstallSettings {