Handle fast-submodules option correctly

This option was introduced in 72cb109bec, but it uses two different
spellings (fast-submodule vs fast-submodules) and isn't handled by
Rust bootstrap which means that any attempt to set this flag fails.

src/bootstrap/bootstrap.py

@@ -647,7 +647,7 @@ class RustBuild(object):
         if (not os.path.exists(os.path.join(self.rust_root, ".git"))) or \
                 self.get_toml('submodules') == "false":
             return
-        slow_submodules = self.get_toml('fast-submodule') == "false"
+        slow_submodules = self.get_toml('fast-submodules') == "false"
         start_time = time()
         if slow_submodules:
             print('Unconditionally updating all submodules')