Reference deapexer directly in bazel-bin

This keeps this test compatible with a Bazel release change which
affects which files are present in `bazel cquery --output=files` for
py_binary targets. Since the executable will be present in a predictable
location under bazel-bin, we can hardcode that, instead.

We could have alternatively used --output_groups=python_zip_file for
'hermetic' python, but the bp2build-built deapexer target is missing
some dependencies on protobuf.

Bug: 265176532
Test: Manual runs of apex_comparison_tests.sh with the new Bazel release
Change-Id: Ib7021e7e81d1a7bf5ff4d222e2a7947a06d16791