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 b2152325ba).
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 https://github.com/actions/setup-java/pull/302
The extendedJavaHome environment variable contains `.` symbols in the version. This causes the environment variable to be ignored by the action runner. It's best to replace those and other non standard symbols with and underscore.
For reference:
> Environment variable names used by the utilities in the Shell and Utilities volume of IEEE Std 1003.1-2001 consist solely of uppercase letters, digits, and the '_' (underscore) from the characters defined in Portable Character Set .
