Allow more time for rustup-init install

Due to rust-lang/rustup#1540, installing the docs on Windows can be
slow enough to time out a fresh rustc install. Marking this `Command`
as `quiet` increases the timeout.

src/tools/rustup.rs

@@ -59,13 +59,16 @@ impl InstallableTool for Rustup {
             native::make_executable(installer)?;
         }
 
+        // TODO(rustup.rs#998): Remove `.quiet(true)` once rust-docs is no longer a mandatory
+        // component.
         RunCommand::new(installer.to_string_lossy().as_ref())
             .args(&[
                 "-y",
                 "--no-modify-path",
                 "--default-toolchain",
                 MAIN_TOOLCHAIN_NAME,
             ])
+            .quiet(true)
             .local_rustup(true)
             .run()
             .with_context(|_| "unable to install rustup")?;