fix: enable to override JAVABIN of wrapper script

[tanishiking]

fix #1431

Description

• This PR enables to override JAVABIN in the wrapper script generated by scala_binary rule
• This behavior aligns to the wrapper script generated by java_binary (sorry I couldn't find where the wrapper script for java_binary defined, but something like)
  • for java_binary: JAVABIN=${JAVABIN:-${JAVA_RUNFILES}/local_jdk/bin/java}

Motivation

please see: #1431

[liucijus]

Thanks, @tanishiking!

[liucijus]

(sorry I couldn't find where the wrapper script for `java_binary` defined, but something like)

The Java rule logic ant the wrapper template seem to be located in this package: https://github.com/bazelbuild/bazel/tree/master/src/main/java/com/google/devtools/build/lib/bazel/rules/java

[simuons]

Thanks @tanishiking

[tanishiking]

The Java rule logic ant the wrapper template seem to be located in this package: https://github.com/bazelbuild/bazel/tree/master/src/main/java/com/google/devtools/build/lib/bazel/rules/java

Thanks!
Oh, looks like java_binary uses := instead of :- (where := fall back to default value for the empty input ''), but it's ok with :- assuming users won't set JAVABIN= (empty).