Automatically download '-baseline' build if CPU does not support AVX2 on MacOS/Windows.

[z0rrn]

The bun-plugin should download the '-baseline' build automatically if it does not detect AVX2 support in the CPU. I already implemented this in #22 on Linux.

• MacOS: This should be easy because, what I think, only the command here must be changed, but I have no Mac and do not understand how to use | in exec_command!.
• Windows: This is way harder because I could not find a command to check for AVX2 support. If anyone has a clue, please help!

If I miss something, just say. It's my first time contributing.
Thanks.