Variables test

[bquan0]

This pull request is a test to see if it is possible to change env variables based on whether the triggering event is a pull_request or a push. It also tests if these changes made in one job will endure into jobs that occur afterwards.

This test is motivated by the virtualbox PR on PyNE, which has the workflows being triggered by pull_request failing because env.DOCKER_IMAGE_TAG changes to something different than the tag of the images in ghcr.io when the triggering event is a pull_request.

This pull request doesn't need to be merged, and I will probably close it after I have worked out all the syntax and nuances necessary. I may also push this variables-test branch onto the cnerg repo instead of my forked repo if the workflow I added is not being run.

[bquan0]

The conclusion of this test is that it is possible to change the env variable DOCKER_IMAGE_TAG if the triggering event is a pull_request. However, the change must be done in every job because the change doesn't carry over to future jobs, even if those future jobs depend on the job where the change occurred.

One more thing to note is that it's not possible to replace github.ref_name with github.head_ref because head_ref only has the branch name when the triggering event is a pull_request.

I will close this PR when the PR in PyNE is closed in case I need to do more testing or refining of this method.

[bquan0]

I'm closing this PR now that we've decided to use other solutions in the PyNE PR which involve not knowing the tag that is assigned to internal images built by the multistage action.