Ask git to checkout a submodule even if it is skipped in `.gitmodules`

[wiktor-k]

Setting a submodule as update = none is a common way to make cargo skip the submodule. Unfortunately this makes actions/checkout skip the submodule too, even if it's configured to fetch submodules.

This patch adds an option that is a default in git but causes the submodules to be available even if skipped otherwise.

See: https://github.com/actions/checkout/issues/915 See: https://github.com/rust-lang/cargo/issues/4247#issuecomment-1149178736 See: https://git-scm.com/docs/git-submodule#Documentation/git-submodule.txt---checkout