Have default 'lunch' target use a release type

In git_main, the correct target here is aosp_arm-trunk_staging-eng
(or "trunk_food" or "next", but we choose the most generic
"trunk_staging").

Bug: 307738446
Test: "lunch", hit return for the default, "echo $TARGET_RELEASE" to confirm
Change-Id: I4d4bcaf27553976e64133c7351db296953f88b08