Use CMake commands to set S2N_LIBCRYPTO environment variable

[boquan-fang]

Problem:

The current way of setting S2N_LIBCRYPTO environment variable is through passing it in CMAKE_PREFIX_PATH, and then use it in CMakeLists.txt. We want to set S2N_LIBCRYPTO in a more deliberate CMake design. https://github.com/aws/s2n-tls/blob/a29068a46ca12a3cb75f559bc2130ac8fa5daa7d/codebuild/spec/buildspec_sanitizer.yml#L104-L109 https://github.com/aws/s2n-tls/blob/a29068a46ca12a3cb75f559bc2130ac8fa5daa7d/CMakeLists.txt#L235-L238

Solution:

• Pass libcrypto into CMake, and use set to set environment variable.

Requirements / Acceptance Criteria:

• Buildspec files shouldn't directly pass in libcrypto through CMAKE_PREFIX_PATH.
• CMakeLists.txt should directly set S2N_LIBCRYPTO via CMake commands.