Use dsim.repos file in GitHub Actions CI

[scpeters]

Our documentation recommends using the dsim.repos to update the workspace, but our GitHub actions CI workflows using a mix of actions/checkout@v2 to clone private dependencies (using the MALIPUT_TOKEN secret) and vcstool to clone public dependencies listed in .github/dependencies.repos. It would be useful to use the dsim.repos file to avoid mistakes like #158 (comment).

Challenges:

• vcstool doesn't currently support https authentication with tokens, though an open pull request proposes support for it in Support authentication dirk-thomas/vcstool#128.
• perhaps the only alternative is adding an ssh key as a secret in this repository (since dsim.repos uses ssh URL's)