Add php 8.0 to GitHub Actions ignoring its errors

[glensc]

The example given in the documentation:

• https://docs.github.com/en/actions/reference/workflow-syntax-for-github-actions#example-including-additional-values-into-combinations

does not actually work as expected:

• Please support something like "allow-failure" for a given job actions/runner#2347

So, this uses the same "experimental: true" setup, but uses the FAILURE_ACTION env variable to skip errors from the experimental PHP configuration (8.0 for now).

cc @falkenhawk

[glensc]

This requires #58 to be merged first, as the ignore error is applied only to the phpunit run.

[falkenhawk]

We should remove the experimental flag in actions/toolkit#32 then, right?

[glensc]

We should remove the experimental flag in actions/toolkit#32 then, right?

For 8.0 yes, but I would keep the "framework" in place for 8.1, or perhaps for 5.4/5.5 as those PHP builds are currently problematic.

The intent of the experimental: true is to say that we support them on a best effort basis, i.e they might work, but might have problems.