Sourcing asdf has broken the zsh

[lngsx]

Recently, I just set up a Linux, Fedora actually, with no homebrew and had
this issue.

I suspect the problem comes from this commit. fc26249

• This code assumes that asdf is already installed in the system, either through
  homebrew or regular asdf.
• This code assumes homebrew already installed in the system.

In a system with no asdf and Homebrew installed, the shell will run into the second case, executing which brew which will cause an error.

I'm not sure what's really going on here. The zsh script is too hardcore for
me. But I believe something's broken when you try to execute brew when it doesn't exist in the system.

My naive implementation would be using the same code introduced in this commit #704.
to check if Homebrew is present in the system before proceed to source the asdf

Thank you.

[lngsx]

Just open the PR to fix this.