fix(uniond): don't statically link on macOS

dynamic on macOS
```
/Users/cor/uno/union $ file ./result/bin/uniond
./result/bin/uniond: Mach-O 64-bit executable arm64
```
static on Linux
```
/home/cor/uno/union $ file ./result/bin/uniond 
./result/bin/uniond: ELF 64-bit LSB executable, ARM aarch64, version 1 (SYSV), statically linked, for GNU/Linux 3.10.0, Go BuildID=avr0bZ6a8YPXsG-fZHfs/d-eZ_UlcDBcJBmh1i5L0/8P6R4fp1kwxETLqKGVI7/zqOCkt5OgCDZGw1933qU, not stripped
```