Don't checkout submodules by default

[sdroege]

This makes sure that cargo does not clone and checkout all the
submodules if pointing to this repository as a git dependency.

To checkout the submodules git submodule update --checkout can be
used.

[sdroege]

Any ideas how this github actions stuff works?

Run actions/checkout@v3
[...]
Adding repository directory to the temporary git global config as a safe directory
/usr/bin/git config --global --add safe.directory /__w/gtk-rs-core/gtk-rs-core
[...]
Run git submodule update --checkout
fatal: unsafe repository ('/__w/gtk-rs-core/gtk-rs-core' is owned by someone else)
To add an exception for this directory, call:

	git config --global --add safe.directory /__w/gtk-rs-core/gtk-rs-core
Error: Process completed with exit code 128.