Document Circleci cache uniqueness /renewal

[tlaurion]

@gaspar-ilom i don't make sense of the circleci error, as if cache was partial at https://app.circleci.com/pipelines/github/gaspar-ilom/heads/14/workflows/b282ff7a-03b4-43af-aed1-369e0454906f/jobs/709

This would need to build without cache to succeed if local builds succeed.

To do this, under Circleci project setting, variable, change/add CACHE_VERSION with unique value (I do datestamp) or change variable name under Circleci config, ie https://github.com/linuxboot/heads/blob/aaeb63df78f8563c46d140f1dcdb51d380392048/.circleci/config.yml#L82

Originally posted by @tlaurion in linuxboot/heads#2007 (comment)