fix: add -Dtarget=x86_64-linux-gnu to remote build
Without this flag, zig links against the host's glibc (2.39 on Ubuntu
24.04), which is newer than the runtime container's (2.36 on Debian
bookworm-slim). The -Dtarget flag makes zig use its bundled libc.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>