Avoid hardcoding host bin path for test artifact

If a developer uses custom out directory, then the tools required for
building the test artifact will not be found in path `out/...`. This CL
removes dependency on the hardcoded path.

Bug: 200300144
Test: `m gen_key_mismatch_capex` with custom out directory
Change-Id: I14197e62d37a583756c1b40025c8c7ed5ab3274d
1 file changed