Add step to verify Homebrew installation to avoid "command not found" error

[karlhorky]

Running brew install ... directly after the Homebrew installation (without running the echo lines mentioned in the instructions) causes the following error message:

$ /bin/bash -c "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/HEAD/install.sh)"

...

$ brew install flyctl git node@18 python
zsh: command not found: brew

Let's research why this happens and make changes to avoid it happening:

• [ ] Research when and why this occurs (maybe there are other scenarios than the one described above)
• [ ] Add a step (or instruction as part of an existing step) to the macOS guide to check for successful Homebrew installation.