bpf_base_test: fix cgroup2 hardcoded path

the path for cgroup2 was moved to its canonical position by
https://r.android.com/c/1324649/5. Move the hardcoded equivalent to the
same path.

Test: presumit tests pass
Bug: 154548692
Change-Id: I50f7a2385b7de88818ac60b0bcd6b92d711e6e16
Merged-In: I50f7a2385b7de88818ac60b0bcd6b92d711e6e16