Use shallow clones for submodules managed by rustbuild, not just bootstrap.py

I missed this in https://github.com/rust-lang/rust/pull/89757; it made
`x.py test src/bootstrap` very slow.

src/bootstrap/lib.rs

@@ -527,7 +527,7 @@ fn dir_is_empty(dir: &Path) -> bool {
         // Try passing `--progress` to start, then run git again without if that fails.
         let update = |progress: bool| {
             let mut git = Command::new("git");
-            git.args(&["submodule", "update", "--init", "--recursive"]);
+            git.args(&["submodule", "update", "--init", "--recursive", "--depth=1"]);
             if progress {
                 git.arg("--progress");
             }