sanitizers: add a merged env target for multi-sanitizer oci_image builds

`score_cpp_policies` provides separate env files for each sanitizer (ASAN, UBSAN, LSAN, TSAN). However, oci_image.env only supports a single env file.

This becomes an issue for valid multi-sanitizer setups like asan_ubsan_lsan, where multiple env files exist but only one can be used. Currently, each consumer works around this by writing a custom genrule to concatenate the env files—I ran into this while integrating sanitizers into the communication repo.

Can we add a `merged_absolute_env` target in `sanitizers/BUILD.bazel` that uses select() over the existing flags (asan_on, ubsan_on, lsan_on, tsan_on) and produces a single merged env file?

This should only cover valid combinations already allowed by the mutually exclusive logic (e.g. asan_ubsan_lsan, tsan, and single-sanitizer cases), not all possible flag combinations.

This would remove duplicated `genrule` logic across downstream repositories.