Currently, git cache depends on the global default branch. Set it to
main explicitly instead.

Bug: 3082645, b/195975120
Change-Id: Ia9331d479dbe9547b6913c74bbf86852fff3a596
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3083251
Auto-Submit: Josip Sokcevic <sokcevic@google.com>
Reviewed-by: Anthony Polito <apolito@google.com>
Commit-Queue: Anthony Polito <apolito@google.com>