build: --baseline flag builds both regular and baseline x64 targets on macOS

Problem

On macOS, the flag in the build script is supposed to build only the baseline (non-AVX) x64 target. However, it currently builds both the regular and baseline targets.

This is a problem for non-AVX Intel Macs (Westmere, Xeon X5690) because:

• The official runtime contains AVX2 instructions and crashes with SIGILL (status 132)
• The binary from Bun 1.3.14 is not actually a baseline build and also contains AVX2
• A patched Bun build using is required for these machines

Expected behavior

should select only the baseline target.

Actual behavior

builds both regular and baseline x64 targets, causing the build to fail or produce an incompatible binary on non-AVX machines.