Make get_build_var and get_abs_build_var into scripts.

Inside envsetup.sh there's an optimization to batch their calls. That's
now done by _get_build_var_cached and _get_abs_build_var_cached.

Bug: 340648588
Test: lunch
Change-Id: I63e39c7dc7705b0e6e7052cb8d9f9840ae398e6b