sbt script: Favor java's `user.home` over `$HOME`
sbt script: Favor java's `user.home` over `$HOME`

JVM powered applications may be given an alternate home by means of the
`user.home` system property, which is handy for managing caches in CI
(the property may be set by different means, including environment
variables such as `JAVA_TOOL_OPTIONS` or `_JAVA_OPTIONS`).

Alas, this doesn't fully work when the `sbt` script downloads the
launcher jar to `$HOME`.

The present change consists in retrieving the value of this property by
means of a `findProperty` function extracted from the existing
`getPreloaded` one (adapted accordingly).

No java process get spawned here.

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