Run CI weekly on `main`

Description

What is this PR

Why is this PR needed?

We'd like to run our tests regularly, because we rarely make changes to the repo, so we are unlikely to catch dependencies going out of date.

What does this PR do?

Adds a GH workflow that triggers the test workflow weekly on Sunday at 3AM only on this repo (not on its forks) and on the main branch.

Closes #55

Unfortunately, newly created scheduled runs are triggered only on the default branch (for security reasons), so this is difficult to test. I suggest to merge this today and check on Monday whether it was successful?

The changes are based on the nightly build of libsbml (where I know it "works").

An earlier attempt #91 at triggering the test workflow from a different workflow was

started because I thought this solution would be more complicated than it turned out.
abandoned because it didn't work out of the box and felt even more complicated in the end.

feels like a waste of energy in case there will be many forks in the future (presumably all ~synced to this one)
it doesn't feel right to cause extra CI runs on other people's repos when they fork/sync their fork

No.

Added a sentence to the README of the CI subfolder.