git hooks are run in an environment without `/usr/local/bin/` on the `PATH`, even though this is always on the `PATH` on macOS, even in a very bare shell (cf `env -i bash --norc --noprofile`).

```

/Applications/Sourcetree.app/Contents/Resources/git_local/libexec/git-core:/Applications/Sourcetree.app/Contents/Resources/bin:/Applications/Sourcetree.app/Contents/Resources/git_local/bin:/Applications/Sourcetree.app/Contents/Resources/git_local/gitflow:/Applications/Sourcetree.app/Contents/Resources/git_local/git-lfs:/usr/bin:/bin:/usr/sbin:/sbin
```

This means that hooks in particular can't use `/usr/bin/env python3`, without having to add `/usr/local/bin/` to the `PATH` themselves.