OWNERS: Change ownership of pkgs-lib

[infinisil]

I'd like to pass off maintainership of pkgs-lib and therefore pkgs.formats to others, so that I can focus on other things. There aren't a lot of PRs for that part of the code, but it adds up.

Here I'm proposing @Stunkymonkey and @h7x4 based on past engagement, let me know if you're okay with that!

Overall I recommend following the lib PR guidelines for maintenance. Note that:

• The docs are easy to forget to update since they're far away from the code (ideally that would be improved).
• Make sure that formats only have generate, type and optionally lib as defined in the docs.

And you're always welcome to ask me if you're unsure about something.

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[Stunkymonkey]

Thanks for the proposal. I am happy to help.