Fix build failure with dlext_testzip Bionic tests

Other build rules create and use a test config file in the intermediate
directory containing $(LOCAL_BUILT_MODULE). Currently, this makefile
deletes $(dir $(LOCAL_BUILT_MODULE)) (aka $(dir $@)), so it can delete
the config file after it's created and before it's used. Avoid this
issue by using $(dir $@)/zipdir as the working directory for
constructing a zip file.

Bug: b/74524170
Test: mmma bionic
Test: run Bionic unit tests
Change-Id: Icee4d7bc72ba1c136385a0cb2c28491649598e7c