Fix banchan for trunk stable builds

In the 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 in places like git_main.

Bug: 307946156
Test: None
Change-Id: Ie418aee2fad192a38beca44cd7d168e0e1b45689