abuild: add postcheck for required license file

Some licenses require distributing a copy of the license file[1], this adds a postcheck to make sure that packages of software with one of these licenses is meeting the requirement.

1. https://wiki.alpinelinux.org/wiki/Creating_an_Alpine_package#license

I think it would be nice to consistently enforce this in aports CI/checks, because right now it's being inconsistently enforced by reviewers who don't always remember to check for this.

A lot of tests use the "MIT" license which causes this postcheck to fail since they don't install a license file ... I can easily fix those tests but I would like to make sure this change is wanted first.

alpine/infra/atools-go!55 (closed) for some previous discussion regarding this

oops I didn't realize this was previously discussed there... Having a check here seems simpler than in atools-go, but this check as it's currently implemented doesn't account for license files that might be installed elsewhere (that was kinda brought up in the MR you linked)?

Yes

nitpick: the license file is not always called LICENSE, sometimes it has an extension (.md, .txt), sometimes it's a different spelling (LICENCE), etc.