toRealPath() is required for untarring Isabelle

26d723e0 broke this, but I have no idea how it ever worked *after* that
change (it probably didn't). The problem is that untarring checks for
"malicious" tarfiles and in the process, normalizes entry paths. If the
outer path is not normalized, the subpath check fails.

src/main/scala/leon/solvers/isabelle/IsabelleEnvironment.scala

@@ -58,7 +58,7 @@ object IsabelleEnvironment {
       case _ =>
         context.reporter.info(s"No $version found at $base")
         context.reporter.info(s"Preparing $version environment ...")
-        Setup.installTo(Files.createDirectories(base), version)
+        Setup.installTo(Files.createDirectories(base).toRealPath(), version)
     }
 
     val system = setup.flatMap { setup =>