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
```

plugins/python-build/bin/python-build

@@ -1537,6 +1537,7 @@ has_broken_mac_openssl() {
 }
 
 use_homebrew_openssl() {
+  command -v brew >/dev/null || return 1
   for openssl in ${PYTHON_BUILD_HOMEBREW_OPENSSL_FORMULA:-openssl}; do
     local ssldir="$(brew --prefix "${openssl}" || true)"
     if [ -d "$ssldir" ]; then