[BUG] Repos are not cloned with submodules

[MattFiler]

Describe the bug When repos are cloned to be built they are not cloned with submodules which results in a build failure for some projects.

Failed Action Log URL (Required) https://github.com/OpenCAGE/CathodeLib/runs/1439021389

To Reproduce Steps to reproduce the behavior:

1. Run the action on a repo with a submodule

Expected Behavior Build without errors.

[Gakk]

I believe you have to specify submodules when running actions/checkout:

steps:
  - uses: actions/checkout@v2
    with:
      submodules: true

From documentation:

Whether to checkout submodules: true to checkout submodules or recursive to recursively checkout submodules.