gitmodules: Disable update to prevent cloning on cargo checkout (#808)

When using a `git` reference on this repo, `cargo` will unnecessarily
clone the `Vulkan-Headers` submodule (which is only needed by maintainers
together with the `generator`).  By setting the update mode to `none`
(https://git-scm.com/docs/gitmodules#Documentation/gitmodules.txt-submoduleltnamegtupdate)
this is disabled, and `git submodule update` will now also no longer
fetch/clone/update the repository unless `--checkout` is used
(https://git-scm.com/docs/git-submodule#Documentation/git-submodule.txt-checkout).

I.e. running `cargo update` on a repo with a `git` dependency on `ash`:

    Updating git repository `https://github.com/ash-rs/ash`
    Skipping git submodule `https://github.com/KhronosGroup/Vulkan-Headers` due to update strategy in .gitmodules

.github/workflows/ci.yml

@@ -32,8 +32,9 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v1
-        with:
+      - name: Checkout submodule
-          submodules: true
+        # Manually update submodules with --checkout because they are configured with update=none and will be skipped otherwise
+        run: git submodule update --recursive --init --force --checkout
       - name: Run generator
         run: cargo run -p generator
       - name: Diff autogen result

.gitmodules

@@ -1,3 +1,4 @@
 [submodule "generator/Vulkan-Headers"]
 	path = generator/Vulkan-Headers
 	url = https://github.com/KhronosGroup/Vulkan-Headers
+	update = none