Add CLA check to all module builds

[SethTisue]

e.g. scala-collection-compat had an admin/checkCLA.sh script that did this, but the details were Travis-CI-specific

I don't know if we were consistent before in doing this in all module repos, but in any case, once all the repos are on GitHub Actions, surely it will be easy to copy Actions code for the CLA check from somewhere — perhaps the Dotty repo, or some Lightbend repo

(does the CLA check exist as its own standalone action, such that it could be added to ci.yml as a one-liner, rather than copy/pasting implementation details? that would be ideal)

[SethTisue]

ah, here's some prior art: scala/vscode-scala-syntax#170

oh but darn:

    - run: ./project/scripts/check-cla.sh

:-/

regardless, a starting point

I should look and see if the copy of the script in vscode-scala-syntax has diverged from the Dotty one, https://github.com/lampepfl/dotty/blob/master/project/scripts/check-cla.sh

[griggt]

I made a change to the Dotty CLA check a few months ago, after GitHub started requiring workflow approval for new contributors. Before the change, the CLA check would run on the user who approved the workflow rather than the user who opened the PR.

[griggt]

Also, I think the Dotty CLA check was originally lifted from https://github.com/scala-js/scala-js/blob/master/ci/check-cla.sh and they may have diverged.