Avoid sbt cache being invalidated for a library that is only incrementing it's own version

While working on https://github.com/guardian/gha-scala-library-release-workflow I noticed that no matter how many times I ran the workflow, `actions/setup-java` would always report `sbt cache is not found`, even if there had been no substantial change in the project - simply that `version.sbt` (the file used by https://github.com/sbt/sbt-release) had the version number in it incremented (as in guardian/play-secret-rotation@b215232).

This meant that turning on `cache: sbt` would actually slow the workflow considerably, as it would never benefit from the cache being present, and would always have to save it, which could take 2-3 minutes - even though it can't take advantage of the data it's saving.

As such, it would be great to exclude `version.sbt` files from the cache hash key.

Background: `cache: sbt` was orginally introduced with #302

Author: rtyley