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>
1 file changed