New tests in test_acvp_vectors.py fail when using custom build path

[iyanmv]

Describe the bug
When passing to cmake a custom build path (e.g. cmake -G Ninja -B build -S liboqs-0.11.0-rc1), the tests in new script test_acvp_vectors.py fail due to FileNotFoundError exceptions. This is because build is harcoded in the calls to helpers.run_subprocess().

To Reproduce
Steps to reproduce the behavior:

1. Build with custom build path
2. Run ninja -C <custom_build_path> run_tests

Expected behavior
All tests should pass

Environment (please complete the following information):

• OS: Arch Linux
• OpenSSL version: 3.3.2
• Compiler version used: GCC 14.2.1
• Build variables used: Like here plus the new variables to enable stateful hash-based signatures.
• liboqs version: 0.11.0-rc1