Fix python-build `brew: command not found` error (#1544)
This PR fixes (currently harmless) warning on macOS when homebrew is *not* installed.
- [ ] Test case (required?)
For example:
```sh
wadkar$ pyenv install 3.8.2
/Users/wadkar/.pyenv/plugins/python-build/bin/python-build: line 1541: brew: command not found
/Users/wadkar/.pyenv/plugins/python-build/bin/python-build: line 1541: brew: command not found
Installing openssl-1.1.0j...
Installed openssl-1.1.0j to /Users/wadkar/.pyenv/versions/3.8.2
Installing readline-8.0...
Installed readline-8.0 to /Users/wadkar/.pyenv/versions/3.8.2
Installing Python-3.8.2...
python-build: use zlib from xcode sdk
Installed Python-3.8.2 to /Users/wadkar/.pyenv/versions/3.8.2
```