CI: Simplify App Checkout

[ax3l]

Summary

Based on API in actions/checkout:
https://github.com/actions/checkout

Additional background

Follow-up to #3348

Checklist

The proposed changes:

• fix a bug or incorrect behavior in AMReX
• add new capabilities to AMReX
• changes answers in the test suite to more than roundoff level
• are likely to significantly affect the results of downstream AMReX users
• include documentation in the code and/or rst files, if appropriate

[WeiqunZhang]

Sure we can use checkout action. But this CI was intended to test the latest release of amrex apps not the latest development branch. So we still need to get the latest tag. Then we could use

    # The branch, tag or SHA to checkout. When checking out the repository that
    # triggered a workflow, this defaults to the reference or SHA for that event.
    # Otherwise, uses the default branch.
    ref: ''

It does not look like there is an easy way to get the latest relase tag. So we may still need to do something like

tag=$(wget https://github.com/AMReX-Astro/Castro/releases/latest 2>&1 | grep Location: | awk '{print $2}' | awk -F/ '{print $NF}')