[CLI] Fix prepare all command

[jhnstn]

This fixes the prepare all command by setting the repo in the build config

Testing

Follow the Testing guide to set up the tool to use forked repos.

• Try go run main.go release prepare all {version}
• Verify that both Gutenberg and Gutenberg Mobile repos are cloned as expected.

[derekblank]

I was able to run go run main.go release prepare all {version} successfully on my forked repos. 🚀