Fix fuzzing workflow: ASAN sanitizer incompatible with musl target

[RAprogramm]

Fix fuzzing workflow failure caused by ASAN sanitizer incompatibility with musl target.

Problem

Fuzzing workflow fails with error:

error: sanitizer is incompatible with statically linked libc, disable it using `-C target-feature=-crt-static`

error[E0463]: can't find crate for `std`
  = note: the `x86_64-unknown-linux-musl` target may not be installed
  = help: consider downloading the target with `rustup target add x86_64-unknown-linux-musl`

Root cause: cargo-fuzz automatically selects x86_64-unknown-linux-musl target which uses static libc linking, incompatible with ASAN sanitizer (-Zsanitizer=address).

Failed run: https://github.com/RAprogramm/cstring-array/actions/runs/18640282197

Solution

Use x86_64-unknown-linux-gnu target instead of musl:

1. Add target installation step
2. Set CARGO_BUILD_TARGET environment variable
3. Or use --target flag in cargo-fuzz command

Implementation

Add to workflow before fuzzing step:

- name: Add gnu target
  run: rustup target add x86_64-unknown-linux-gnu

- name: Run fuzzer
  run: cargo fuzz run ${{ matrix.target }} --target x86_64-unknown-linux-gnu -- -max_total_time=300

Benefits

• Fuzzing works with ASAN sanitizer
• Better memory safety testing
• CI passes successfully