pw_env_setup: Capture CIPD stderr

Prevent CIPD from spewing output to stderr unless it fails.

Change-Id: I9a377615f43b5e9883948bc13748c0553b1c7ac7
diff --git a/pw_env_setup/py/pw_env_setup/cipd_setup/update.py b/pw_env_setup/py/pw_env_setup/cipd_setup/update.py
index 3dfb9d5..e64b3a8 100755
--- a/pw_env_setup/py/pw_env_setup/cipd_setup/update.py
+++ b/pw_env_setup/py/pw_env_setup/cipd_setup/update.py
@@ -146,7 +146,9 @@
         with tempfile.TemporaryFile(mode='w+') as temp:
             print(*cmd, file=temp)
             try:
-                subprocess.check_call(cmd, stdout=temp)
+                subprocess.check_call(cmd,
+                                      stdout=temp,
+                                      stderr=subprocess.STDOUT)
             except subprocess.CalledProcessError:
                 temp.seek(0)
                 sys.stderr.write(temp.read())