Open temporary cookie file as writable in sync.py

Named Temporary file defaults to mode 'w+b' which causes repo sync to
fail. By opening the tmpcookiefile in PersistentTransport.request as
writable, we are able to run sync successfully.

Bug: https://crbug.com/gerrit/12370
Test: Ran smartsync successfully
Change-Id: I01ddf915fc30eb3ff0e4d440a6f1aa261c63e88d
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/255692
Tested-by: Jonathan Nieder <jrn@google.com>
Reviewed-by: Jonathan Nieder <jrn@google.com>