Externalise cache keys to prevent unneeded invalidation

[pljones]

Short description of changes

I noticed whilst I've been working on autobuild.sh that every change invalidates all the caches. Because autobuild.sh itself is included in the cache key calculation. This change replaces that mechanism on macOS and Windows with am externalised list of dependencies. It adds the same mechanism to iOS, which lacked caching.

Github cache clean up will remove unused cache entries with no need to have the cache consumer included in the cache key.

(My work in progress on the Android build uses the same approach, which is where this comes from.)

Scope, guidance, testing and review by myself, coding my VSCode Github CoPilot.

CHANGELOG: Build: improve cache persistence

Context: Fixes an issue?

Caching should depend on what is being cached, rather than the cache consumer.

Does this change need documentation? What needs to be documented and how?

No.

Status of this Pull Request

Tested against the current branch in my own repository before raising the PR here.

What is missing until this pull request can be merged?

Need to do some more testing.

Checklist

• I've verified that this Pull Request follows the general code principles
• I tested my code and it does what I want
• My code follows the style guide
• I waited some time after this Pull Request was opened and all GitHub checks completed without errors.
• I've filled all the content above

AUTOBUILD: Please build all targets