Set swoval.tmpdir property on startup

This change makes the temporary shared library that is created by
the swoval file-tree-views library to be extracted into the sbt global
base directory rather than the temp file. This way if there is a leak of
shared libraries, they can easily be found in ~/.sbt rather than in,
say, /tmp (or the osx/windows equivalent location). The extracted shared
library objects will be in the path ~/.sbt/swoval-jni. There is a
shutdown hook that removes them as well as a garbage collection process
that runs in the background whenever the swoval library is loaded, so
these shouldn't leak uncontrollably.

main/src/main/scala/sbt/internal/Load.scala

@@ -113,6 +113,7 @@ private[sbt] object Load {
     val delegates = defaultDelegates
     val pluginMgmt = PluginManagement(loader)
     val inject = InjectSettings(injectGlobal(state), Nil, const(Nil))
+    System.setProperty("swoval.tmpdir", System.getProperty("swoval.tmpdir", globalBase.toString))
     LoadBuildConfiguration(
       stagingDirectory,
       classpath,