Adding ApPredict GitHub Actions workflow on self-hosted Ubuntu 20.04 runners

[kwabenantim]

Description

• This PR adds a GitHub Actions workflow to build and run the ApPredict project test pack with a self-hosted runner on Ubuntu 20.04.
• The workflow, (1) is scheduled to run every midnight, (2) is triggered when a new commit is made to the ApPredict repository, and (3) can be triggered manually.

Motivation and Context

• This PR is part of Chaste ticket 3099 (Migrate CI from Buildbot to GitHub actions ).
• For existing GitHub Actions workflows on the Chaste repository, see Chaste/Chaste/.github/workflows.

Testing

• This GitHub Actions workflow has been tested on a fork.

Additional Notes

• A self-hosted runner should be added, preferably at the organization level so multiple repositories can share it: Adding a self-hosted runner to an organization (this will generate a token that is required for configuring the runner).
• This workflow assumes that self-hosted runners have the required dependencies for building and testing Chaste pre-installed.

[MauriceHendrix]

Overall the alooks good. I'm just wondering:

• Can you add the docker file for the runner as well?
• Maybe is it possible to set the number of processes in a different way instead of hard coding? Maybe an environment variable, or something?

[mirams]

Looks good to me, safe to give it a try I think?