Build skottie_tool on Linux only

MoltenVK builds are failing on Mac.  Let's limit skottie_tool to Linux
for now, as that's the only platform Fiddle runs on.

Change-Id: Icc94e0e380d6f1bc32888dcaaeb812dad082f625
Reviewed-on: https://skia-review.googlesource.com/132404
Commit-Queue: Florin Malita <fmalita@chromium.org>
Commit-Queue: Hal Canary <halcanary@google.com>
Auto-Submit: Florin Malita <fmalita@chromium.org>
Reviewed-by: Hal Canary <halcanary@google.com>
diff --git a/BUILD.gn b/BUILD.gn
index 4c53f0c..85d6694 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -1514,7 +1514,7 @@
     ]
   }
 
-  if (is_linux || is_mac || is_win) {
+  if (is_linux) {
     test_app("skottie_tool") {
       deps = [
         "modules/skottie:tool",