Don't checkout rust submodule by default

It is no longer necessary for regular builds as of the previous commit.
This will drastically improve the time it takes to do the initial clone
of redox.

.gitmodules

@@ -10,6 +10,7 @@
 	path = rust
 	url = https://gitlab.redox-os.org/redox-os/rust.git
 	branch = redox-2023-09-07
+	update = none
 [submodule "redoxfs"]
 	path = redoxfs
 	url = https://gitlab.redox-os.org/redox-os/redoxfs.git

mk/prefix.mk

@@ -73,6 +73,9 @@ $(PREFIX)/rust-install: $(PREFIX)/rust-install.tar.gz
 
 else
 
+$(ROOT)/rust:
+	git submodule update --init --recursive --checkout rust
+
 PREFIX_BASE_INSTALL=$(PREFIX)/rust-freestanding-install
 PREFIX_FREESTANDING_INSTALL=$(PREFIX)/gcc-freestanding-install