Support alternative sbt-launch repo
Support alternative sbt-launch repo

Problem
-------
Not everyone has direct access to Maven Central.

Solution
--------
Provide an env variable to override the base URL for sbt launcher.

sbt/sbt,mdedetrich/sbt,mdedetrich/sbt,xuwei-k/xsbt,sbt/sbt,sbt/sbt,mdedetrich/sbt,xuwei-k/xsbt,xuwei-k/xsbt,sbt/sbt,sbt/sbt,mdedetrich/sbt,sbt/sbt,mdedetrich/sbt,xuwei-k/xsbt,mdedetrich/sbt,sbt/sbt,mdedetrich/sbt,xuwei-k/xsbt