Add the 'always-cache' option

[kminehart]

Description:
The always-cache option will always update the cache, even in the event of a cache hit. This is particularly useful for larger projects with a lot of unit tests and a high volume of changes.

Related issue:
#569

Check list:

• Mark if documentation changes are required.
• Mark if tests were added or updated to cover the changes.

[kminehart]

Ah. I just tested this and realized that this won't work; it will fail to save the cache with a key that already exists, and @actions/cache has no function to delete or overwrite a cache key.