Use a /var/tmp path for Bazel output tree on OSX

The current value:
1) Isn't created by anything, and
2) Generates too long a path for the name of a Unix domain socket

--
MOS_MIGRATED_REVID=89152182

src/main/cpp/blaze_util_darwin.cc

@@ -30,11 +30,7 @@ namespace blaze {
 using std::string;
 
 string GetOutputRoot() {
-  const char *home = getenv("HOME");
-  if (home == NULL) {
-    return "/var/tmp";
-  }
-  return blaze_util::JoinPath(home, "Library/Caches/Bazel");
+  return "/var/tmp";
 }
 
 void WarnFilesystemType(const string& output_base) {