Ubuntu setup - additional steps required to install homebrew

In the [Linux setup instructions](https://carpentries.github.io/lesson-example/setup.html#linux-ubuntu), the first step is:

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

A few things to note after this step that may be useful to include in the instructions:

1. This may require admin password
2. There are three commands listed in `stdout` that you must follow. This is what that looks like on my machine (my output explicitly defined my home dir; I've replaced that with `~` here):

```
==> Next steps:
- Run these three commands in your terminal to add Homebrew to your PATH:
echo '# Set PATH, MANPATH, etc., for Homebrew.' >> ~/.bash_profile
echo 'eval "$(/home/linuxbrew/.linuxbrew/bin/brew shellenv)"' >> ~/.bash_profile
eval "$(/home/linuxbrew/.linuxbrew/bin/brew shellenv)"
- Install Homebrew's dependencies if you have sudo access:
sudo apt-get install build-essential
For more information, see:
https://docs.brew.sh/Homebrew-on-Linux
- We recommend that you install GCC:
brew install gcc
- Run brew help to get started
```