Do not check ssvnc for Darwin user.

This CL enables running acloud without prompting apt-get install ssvnc.

Bug: 149073595

Test: In MacOS:
   m acloud && acloud-dev create

Change-Id: I3dd8593bdfbdac3bb49ba6e096afc88665108718
diff --git a/internal/lib/utils.py b/internal/lib/utils.py
index d19d448..87c6301 100755
--- a/internal/lib/utils.py
+++ b/internal/lib/utils.py
@@ -922,7 +922,7 @@
                          "Skipping VNC startup.", TextColors.FAIL)
         return
 
-    if not FindExecutable(_VNC_BIN):
+    if IsSupportedPlatform() and not FindExecutable(_VNC_BIN):
         if no_prompts or GetUserAnswerYes(_CONFIRM_CONTINUE):
             try:
                 PrintColorString("Installing ssvnc vnc client... ", end="")