avoid updating submodules during bootstrap unit tests

Unit tests use their own config values instead of reading user's config file,
which can lead to submodule fetching which is not needed for the tests.

Signed-off-by: onur-ozkan <work@onurozkan.dev>

Author: onur-ozkan

src/bootstrap/src/core/config/config.rs

@@ -2526,7 +2526,7 @@ impl Config {
     /// used instead to provide a nice error to the user if the submodule is
     /// missing.
     pub(crate) fn update_submodule(&self, relative_path: &str) {
-        if !self.submodules() {
+        if !self.submodules() || cfg!(feature = "bootstrap-self-test") {
             return;
         }