toolchain-utils: create pending_archives if needed

The change fixes the following error happening after migration to a new
chrotomation:
cp: cannot create regular file '/usr/local/google/crostc/pending_archives/.': No such file or directory
Traceback (most recent call last):
  ...
  File "./buildbot_test_toolchains.py", line 259, in _CopyJson
    raise RuntimeError(
RuntimeError: Crosperf failed to run tests, cannot copy json report!

BUG=None
TEST=To be tested on chrotomation.mtv.

Change-Id: I4de2bb1ec816fa6d9e98993d6da1b6bded34b1f5
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/toolchain-utils/+/2453594
Commit-Queue: Denis Nikitin <denik@chromium.org>
Tested-by: Denis Nikitin <denik@chromium.org>
Reviewed-by: George Burgess <gbiv@chromium.org>
diff --git a/buildbot_test_toolchains.py b/buildbot_test_toolchains.py
index e4784a6..4898796 100755
--- a/buildbot_test_toolchains.py
+++ b/buildbot_test_toolchains.py
@@ -250,6 +250,8 @@
       self._ce.RunCommand(command)
 
   def _CopyJson(self):
+    # Make sure a destination directory exists.
+    os.makedirs(PENDING_ARCHIVES_DIR, exist_ok=True)
     # Copy json report to pending archives directory.
     command = 'cp %s/*.json %s/.' % (self._reports_dir, PENDING_ARCHIVES_DIR)
     ret = self._ce.RunCommand(command)