stick to 10.14 Mac bots

XCode license agreement isn't working on 10.15.

Change-Id: I5f80868e427c0d3f6aa41005c66e9dee4350eea5
Reviewed-on: https://skia-review.googlesource.com/c/skcms/+/259827
Reviewed-by: Ben Wagner aka dogben <benjaminwagner@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
diff --git a/infra/bots/tasks.json b/infra/bots/tasks.json
index 04fe680..78b9ead 100644
--- a/infra/bots/tasks.json
+++ b/infra/bots/tasks.json
@@ -108,7 +108,7 @@
         "cache/Xcode_skcms.app"
       ],
       "dimensions": [
-        "os:Mac",
+        "os:Mac-10.14.6",
         "gpu:none",
         "pool:Skia"
       ],