dev-desktop: automatically add git config username&email for all gh users

[oli-obk]

We can grab all this information from their profile (using the github email address that anonymizes your actual email address)

[Mark-Simulacrum]

Many contributors don't use a github-anonymized email address today, and today I believe would get an error on first commit, whereas in the future would silently use the 'wrong' email; it's likely to add a lot of churn to our mailmap file (or would need support in rust-lang/thanks) if we do this.

[oli-obk]

we could scrape the info from the mailmap? XD

[pietroalbini]

Also see https://github.com/rust-lang/simpleinfra/pull/110

Not sure how much of a problem users manually setting it would be though.

[jdno]

https://github.com/rust-lang/simpleinfra/pull/110 has been merged and setting the Git username and email is now part of the init.sh script.