luci: update xcode cache version.

Tbr: justincohen@chromium.org
Bug: 903674
No-Try: true
Change-Id: Icc14807608fbd7eac94a129a6c0a6b41b187d34d
Reviewed-on: https://chromium-review.googlesource.com/c/1331828
Commit-Queue: John Budorick <jbudorick@chromium.org>
Reviewed-by: John Budorick <jbudorick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607308}

infra/config/global/cr-buildbucket.cfg

@@ -439,9 +439,9 @@ builder_mixins {
 builder_mixins {
   name: "xcode-caches"
   caches: {
-    # Cache for Xcode 10 (build version 10a254a) needed for iOS builds.
-    name: "xcode_ios_10a254a"
-    path: "xcode_ios_10a254a.app"
+    # Cache for Xcode 10.1 (build version 10b61) needed for iOS builds.
+    name: "xcode_ios_10b61"
+    path: "xcode_ios_10b61.app"
   }
 }