Export sbt environment variables in 'setup' stage

In #154, export of SBT_OPTS and JVM_OPTS were wrongly added to 'export'
stage, which is executed before 'git clone'.
'setup' stage is then used, similarly to `jdk_switcher` call in jdk.rb
Merge pull request #174 from gildegoma/fix-scala-exports

Export sbt environment variables in 'setup' stage