Swarming: Ensure depot_tools is in PATH when compiling

Turns out we were "lucky" for our Linux bots which already have depot_tools in PATH. The Mac bot does have depot_tools, but it's not in PATH.  Add it in the script to ensure we can always get to it.

NOTRY=true
BUG=skia:4763
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1823653002

Review URL: https://codereview.chromium.org/1823653002
diff --git a/infra/bots/flavor/default_flavor.py b/infra/bots/flavor/default_flavor.py
index 4d11117..e598873 100644
--- a/infra/bots/flavor/default_flavor.py
+++ b/infra/bots/flavor/default_flavor.py
@@ -107,6 +107,9 @@
       env['CHROME_PATH'] = self.chrome_path
       self._bot_info._run_once(self.bootstrap_win_toolchain)
     else:
+      # Ensure depot_tools is in PATH.
+      depot_tools = os.path.join('/b', 'depot_tools')
+      env['PATH'] = os.pathsep.join([depot_tools, os.environ['PATH']])
       make_cmd = ['make']
     cmd = make_cmd + [target]
     self._bot_info.run(cmd, env=env)