Fix tapas for git_main builds

In the git_main trunk stable builds, we're required to have a
TARGET_RELEASE set.

For now, we hard-code this to "trunk_staging" (and leave a TODO
to accept this as a function argument).

This will have us producing valid builds under git_main.

Bug: 307975293
Test: 'tapas google-tradefed-all' now works on a tree that has go/aog/2806253
Change-Id: I70572fb4c7902350513e251694f3ddef91efcb13