Add LLVM to the upstream jail

[elisehaldane]

Pull Request

Checklist

• Have you read Digital Catapult's Code of Conduct?
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.

PR Type

Please delete options that are irrelevant.

• Feature

High level description

This change adds the LLVM package to the upstream jail, to reduce the time taken for a workflow to finish.

Describe alternatives you've considered

GitHub caching might be an option, but adding packages directly to a jail is more in keeping with the design of Pot itself and its objective of offering containerisation within FreeBSD. A cache couldn't be exported along with the jail, when running pot export, hence the build artefact wouldn't be an accurate recreation of the original state. As a result, installing packages with a view to exporting the jail as a tarball is still the preferred approach.