macOS is unsupported.

[benreyn]

Describe the bug

asdf-godot is not supported on macOS systems, but fails silently

Steps to reproduce

On a macOS based system...

$ asdf install godot latest
* Downloading godot release 3.4.2...
godot 3.4.2 installation was successful!

$ asdf global godot latest

$ godot --version
/usr/local/opt/asdf/libexec/lib/commands/command-exec.bash: line 28: /Users/username/.asdf/installs/godot/3.4.2/bin/godot: cannot execute binary file
/usr/local/opt/asdf/libexec/lib/commands/command-exec.bash: line 28: /Users/username/.asdf/installs/godot/3.4.2/bin/godot: Undefined error: 0

$ file /Users/username/.asdf/installs/godot/3.4.2/bin/godot
/Users/username/.asdf/installs/godot/3.4.2/bin/godot: ELF 64-bit LSB executable, x86-64, version 1 (GNU/Linux), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, for GNU/Linux 5.9.0, stripped

Expected behavior

asdf-godot is operating system aware, and installs mac specific builds if it is run on a Darwin based system.