finalize_branch_for_release.sh: avoid envsetup.sh

Avoid envsetup.sh, which is configuring the environment in a
way that breaks builds on the build servers. This exposes an
issue with the AIDL freeze that was lost in the meantime.

Bug: 234734342
Test: (run script, there is an audio failure)
Change-Id: Ic2888fb316d295a06f465f33d3a6f348a65c30a5