fix(init): use yarn as package manager when calling init with npx

[szymonrybczak]

Summary:

Fixes#2213, when using npx -> npm_config_user_agent is npm.

Test Plan:

Not really tested, reverts old behaviour when user has strange setup.

Checklist

• Documentation is up to date to reflect these changes.
• Follows commit message convention described in CONTRIBUTING.md

[szymonrybczak]

I think we can backport this one cc @thymikee

[thymikee]

It's on the 12.x branch now, we'll include it in the next minor bump