chore: Do not lint external apps

[ChristophWurst]

Summary

Running npm run lint on master scans all apps and therefore takes a long time and might format apps in unwanted ways. Moreover, if there is an incompatible eslint configin one of the apps, like Contacts, the linting errors out after ~30s.

$ npm run lint

> [email protected] lint
> eslint apps core


Oops! Something went wrong! :(

ESLint: 8.46.0

Error: apps/contacts/.eslintrc.js » @nextcloud/eslint-config/typescript:
        Configuration for rule "n/no-unpublished-import" is invalid:
        Value {"allowModules":["@vue/test-utils","@testing-library/vue"],"ignoreTypeImport":false} should NOT have additional properties.

By using a short script we can build a list of apps that are part of the server repo. Lint now finishes in ~11s on my machine.

The same trick is also used for sass: #38590.

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)