[1.3.0] Double dashes for sbt-launcher don't work on Darwin

[anilkumarmyla]

steps

sbt version: 1.3.0

problem

If we pass in double dashed option --sbt-version, it doesn't work correctly
Debug info

$ sbt -debug --sbt-version 1.3.0 update
[addSbt] arg = '-debug'
[residual] arg = '--sbt-version'
[residual] arg = '1.3.0'
[residual] arg = 'update'
[residual] arg = '--sbt-version'
[residual] arg = '1.3.0'
[residual] arg = 'update'
[process_args] java_version = '12'
[sbt_options] declare -a sbt_options='()'
OpenJDK 64-Bit Server VM warning: Ignoring option MaxPermSize; support was removed in 8.0
[copyRt] java9_rt = '/Users/anilkumarmyla/.sbt/0.13/java9-rt-ext-adoptopenjdk_12_0_2/rt.jar'
[addJava] arg = '-Dscala.ext.dirs=/Users/anilkumarmyla/.sbt/0.13/java9-rt-ext-adoptopenjdk_12_0_2'
# Executing command line:
java
-XX:+UseParallelGC
-Dfile.encoding=UTF-8
-Xms8192m
-Xmx8192m
-Xss4M
-XX:ReservedCodeCacheSize=512m
-XX:MaxPermSize=1024m
-Dscala.ext.dirs=/Users/anilkumarmyla/.sbt/0.13/java9-rt-ext-adoptopenjdk_12_0_2
-jar
/usr/local/Cellar/sbt/1.3.0/libexec/bin/sbt-launch.jar
-debug
--sbt-version
1.3.0
update

When passed as single dash, it works correctly by passing -Dsbt.version=1.3.0

$ sbt -debug -sbt-version 1.3.0 update
[addSbt] arg = '-debug'
[residual] arg = 'update'
[residual] arg = 'update'
[addJava] arg = '-Dsbt.version=1.3.0'
[process_args] java_version = '12'
[sbt_options] declare -a sbt_options='()'
OpenJDK 64-Bit Server VM warning: Ignoring option MaxPermSize; support was removed in 8.0
[copyRt] java9_rt = '/Users/anilkumarmyla/.sbt/0.13/java9-rt-ext-adoptopenjdk_12_0_2/rt.jar'
[addJava] arg = '-Dscala.ext.dirs=/Users/anilkumarmyla/.sbt/0.13/java9-rt-ext-adoptopenjdk_12_0_2'
# Executing command line:
java
-XX:+UseParallelGC
-Dfile.encoding=UTF-8
-Xms8192m
-Xmx8192m
-Xss4M
-XX:ReservedCodeCacheSize=512m
-XX:MaxPermSize=1024m
-Dsbt.version=1.3.0
-Dscala.ext.dirs=/Users/anilkumarmyla/.sbt/0.13/java9-rt-ext-adoptopenjdk_12_0_2
-jar
/usr/local/Cellar/sbt/1.3.0/libexec/bin/sbt-launch.jar
-debug
update

expectation

Double dashed option should work correctly as advertized in sbt -help

[eed3si9n]

Fixed in sbt/sbt-launcher-package#281