Let users specify a different location for build repos than user home

This change is helpful e.g. when setting up CI for Scala and it's important which directories are used by a build.
By default it uses .m2, .pax and .sbt/cache from user home (in general $HOME). Sometimes it's not possible to use
this location and also changing a value of $HOME is not an option.

The required location should be specified both for ant's build.xml and used scripts.
In the first case specifying -Duser.home is all, what we need. But we can't just set _JAVA_OPTIONS
as then partest tests fail due to the unexpected output (an additional 'Picked up java options (...)' messages).
We can set ANT_OPTS instead of this. The only problem was that OSGi JUnit tests (only they) were using $HOME anyway.
I forced them to use -Duser.home by propagating this option in build.xml.

binary-repo-lib.sh is changed to use a special env variable or $HOME, when this variable is not set.

Then we can do:

export SCALA_BUILD_REPOS_HOME=<some_dir>
export ANT_OPTS="$ANT_OPTS -Duser.home=$SCALA_BUILD_REPOS_HOME"
ant dist

and it will create all directories .m2, .pax and .sbt/cache in a specified $SCALA_BUILD_REPOS_HOME directory.

Note: maven's settings.xml should be located in this used $SCALA_BUILD_REPOS_HOME/.m2 and point to the
repository like <some_dir>/.m2/repository
Merge pull request #4130 from mpociecha/let-specify-build-repos-home

Let users specify a different location for build repos than user home