Don't checkout git submodules by default

.gitlab-ci.yml

@@ -29,6 +29,7 @@ docs:
   stage: test
   variables:
     GIT_SUBMODULE_STRATEGY: recursive
+    GIT_SUBMODULE_UPDATE_FLAGS: --checkout
   script:
     # generate the docs
     - cargo install rustdoc-stripper

.gitmodules

 [submodule "gir"]
 	path = gir
 	url = https://github.com/gtk-rs/gir.git
+	update = none
 [submodule "gir-files"]
 	path = gir-files
 	url = https://github.com/gtk-rs/gir-files.git
+	update = none