[infra] Ensure Build-Mac backfill tasks can find Xcode.

This CL prepares for installing Xcode in a Swarming cache dir from a
CIPD package. https://skia-review.googlesource.com/c/skia/+/144608

Change-Id: I123a409f97397cc0e87de0dcd05d7ea08d3c0524
Reviewed-on: https://skia-review.googlesource.com/144903
Commit-Queue: Eric Boren <borenet@google.com>
Auto-Submit: Ben Wagner <benjaminwagner@google.com>
Reviewed-by: Eric Boren <borenet@google.com>

infra/bots/recipe_modules/build/default.py

@@ -63,6 +63,13 @@ def compile_fn(api, checkout_root, out_dir):
   args = {}
   env = {}
 
+  if os == 'Mac':
+    # In the future we will use CIPD to install Xcode on Mac builders. If we are
+    # backfilling after a task that used XCode from CIPD, we need to select the
+    # default Xcode again.
+    api.step('select xcode', [
+        'sudo', 'xcode-select', '-switch', '/Applications/Xcode9.2.app'])
+
   if compiler == 'Clang' and api.vars.is_linux:
     cc  = clang_linux + '/bin/clang'
     cxx = clang_linux + '/bin/clang++'

infra/bots/recipe_modules/build/examples/full.expected/Build-Mac-Clang-arm64-Debug-iOS.json

@@ -1,4 +1,13 @@
 [
+  {
+    "cmd": [
+      "sudo",
+      "xcode-select",
+      "-switch",
+      "/Applications/Xcode9.2.app"
+    ],
+    "name": "select xcode"
+  },
   {
     "cmd": [
       "python",

infra/bots/recipe_modules/build/examples/full.expected/Build-Mac-Clang-x86_64-Debug-CommandBuffer.json

@@ -1,4 +1,13 @@
 [
+  {
+    "cmd": [
+      "sudo",
+      "xcode-select",
+      "-switch",
+      "/Applications/Xcode9.2.app"
+    ],
+    "name": "select xcode"
+  },
   {
     "cmd": [
       "python",

infra/bots/recipe_modules/build/examples/full.expected/Build-Mac-Clang-x86_64-Debug-Metal.json

@@ -1,4 +1,13 @@
 [
+  {
+    "cmd": [
+      "sudo",
+      "xcode-select",
+      "-switch",
+      "/Applications/Xcode9.2.app"
+    ],
+    "name": "select xcode"
+  },
   {
     "cmd": [
       "python",

infra/bots/recipe_modules/build/examples/full.expected/Build-Mac-Clang-x86_64-Release-MoltenVK_Vulkan.json

@@ -1,4 +1,13 @@
 [
+  {
+    "cmd": [
+      "sudo",
+      "xcode-select",
+      "-switch",
+      "/Applications/Xcode9.2.app"
+    ],
+    "name": "select xcode"
+  },
   {
     "cmd": [
       "python",

infra/bots/recipes/compile.expected/Build-Mac-Clang-x86_64-Debug-CommandBuffer.json

@@ -127,6 +127,15 @@
     "infra_step": true,
     "name": "makedirs tmp_dir"
   },
+  {
+    "cmd": [
+      "sudo",
+      "xcode-select",
+      "-switch",
+      "/Applications/Xcode9.2.app"
+    ],
+    "name": "select xcode"
+  },
   {
     "cmd": [
       "python",