cache clean workflow

[alan-forbes-cp]

Overview

Workflow to automatically clean caches by schedule (with manual option).

Reason for change

Cache housekeeping (size limit exists).

Description of change

Cache clean workflow rationale:

• uses gh command line tool functionality
• runs on ubuntu-latest
• needs GH_TOKEN & 'action: write' perms (for gh actions-cache delete)
• schedules cache clean every night @ 23:00hrs
• workflow dispatch is also included - with dryrun option (default "true")
• cleans cache for all items contained in cache prefix list.
• cleans main branch only
• retains latest cached item for each listed prefix
• produces before/during/after logging

Anything else we should know?

Note that caching in CodeQL has been disabled (trap-caching: false added in workflow) as closer inspection of this feature has unearthed an apparent bug (a single cache item name being used for multiple jobs in the CodeQL workflow causing "cache already exists" warnings) together with support's recent suggestion that the feature be disabled due to other caching issues.

Some code has been added for testing purposes and will be removed in the final version - see comments.