The computation around `FULL_SBT_VERSION` is messy. With these changes, the caching mechanism when rebuilding from Scala to Scala IDE locally is not working anymore. A value is set at the beginning which will contain an git hash of Scala and this value is used to check if it needs to be rebuild. And then the value is overwritten with the content of the properties file, and this new value used to build and store the binaries in the local m2 repo.
If the same build is run again, the availability check will fail because the binaries were store with a different version number.

The use cases are:
- Scala pr-validation - caching (but not critical, almost never used in practice)
- local rebuild Scala to Scala IDE - caching
- building for nightly validation - no caching
- nightly build to deploy on artifactory for normal developers local builds - no caching
- building a 'release' of zinc - no caching (not supported, might not be needed with next point)
- building for Scala IDE release - caching (not supported yet)

I'll sleep on the problem, and see I get a bright idea.