LUCI: Enable git superproject checkout for QEMU (non-repo)

Long-term isolated test whether this approach works.

Unlike repo, this would:
 - Actually guarantee to checkout the relevant CL rather than ToT
 - Ensure all builders test the same code
 - Be able to checkout older CLs for retries or binary search
 - Avoid any split topic issues

Test: trigger build with LUCI led
Change-Id: Ie3a490cbfdd5f76834089d26fa64de6e7e31abea