Currently, the PowerShell execution does not allow STDIN to be
read by the CIPD client. Fix this by using PowerShell to acquire
the client, but batch to actually invoke the client. This removes
some indirection and allows STDIN to function as expected.

BUG=None
TEST=local
  - Ran w/ and w/out ".cipd_client.exe" locally, observed
    install and fallthrough.
  - Ran w/ an error code, observed exit code being preserved.
  - Ran "auth-login", was able to paste token.

Change-Id: I4efafa7dc80aa093de0fbf5dfd188c299fa104db
Reviewed-on: https://chromium-review.googlesource.com/563602
Commit-Queue: Daniel Jacques <dnj@chromium.org>
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>