Respect custom Cargo target dir in JNI build

[dannyward630]

Summary

• resolve Cargo's actual target directory with cargo metadata
• copy desktop/server JNI libraries from that resolved directory instead of hard-coded target/...
• keep Android artifact handling unchanged because it already uses --artifact-dir

Why

java/build_jni.sh respected Cargo's configured target directory during the build, but then copied desktop/server libraries from target/$triple/.... That breaks users who set CARGO_TARGET_DIR or configure build.target-dir, as reported in #655.

Validation

• bash -n bin/build_helpers.sh java/build_jni.sh
• git diff --check
• fake-toolchain smoke: ran java/build_jni.sh desktop with a fake cargo/rustc and CARGO_TARGET_DIR set to a temp custom directory; the script copied libsignal_jni_amd64.dylib and libsignal_jni_testing_amd64.dylib from that custom target directory into java/client/src/main/resources

I could not run the full Rust/JNI build in this local environment because cargo is not installed on this machine.

Fixes #655.