Update development-guide to include installation of `Packer`

# 💡 Summary #

Add steps to install [packer](https://www.packer.io/) as so users can take advantage of the `setup-env` script.

## Motivation and context ##

We should include some info in our dev guide about installing [packer](https://www.packer.io/). The version should be pinned to the version specified in [cisagov/setup-env-github-action](https://github.com/cisagov/setup-env-github-action). Right now unless you run the [laptop script](https://github.com/cisagov/laptop) instructed in the [mac-env-setup ](https://github.com/cisagov/development-guide/blob/develop/dev_envs/mac-env-setup.md) users will run into an error. It can simply be fixed after running

```console
brew install packer
```

## Acceptance criteria ##

How do we know when this work is done?

- [ ] Development guide includes steps about manually installing packer