Checkout public submodules from gitlab (or others)

[ganondev]

My repository (https://github.com/ganondev/libant) depends on https://gitlab.com/libeigen/eigen as a submodule. I thought to recurse submodules as part of the checkout:

    steps:
      - uses: actions/checkout@v3
        with:
          submodules: recursive

But I get this:

  Submodule 'submodules/eigen' ([email protected]:libeigen/eigen.git) registered for path 'submodules/eigen'
  Cloning into 'D:/a/libant/libant/submodules/eigen'...
  Host key verification failed.

Given that this is a public repository, and GitLab isn't exactly an obscure service, I feel like this ought to work out of the box. I didn't have any luck trying to add gitlab's key to ssh-known-hosts, so I'm not sure what to do.