Update development-guide to include installation of `Packer`

[michaelsaki]

💡 Summary

Add steps to install packer 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. The version should be pinned to the version specified in cisagov/setup-env-github-action. Right now unless you run the laptop script instructed in the mac-env-setup users will run into an error. It can simply be fixed after running

brew install packer

Acceptance criteria

How do we know when this work is done?

• Development guide includes steps about manually installing packer