CI: fix appstream linting

[daringer]

The linting step currently fails as cloudflare seems to block the action's get call to nitrokey.com with 403. No easy fix currently, it should be fine to disable this test

[mmerklinger]

The linting doesn't seem to be broken anymore, but gives a warning.

I: com.nitrokey.nitrokey-app2:48: developer-name-tag-deprecated
   The toplevel `developer_name` element is deprecated. Please use the `name` element in a
   `developer` block instead.

I: com.nitrokey.nitrokey-app2:~: developer-info-missing
   This component contains no `developer` element with information about its author.

✔ Validation was successful: infos: 2

I think we should change this before it becomes an error.

[daringer]

Funny though this seems to be not deterministic - looks like some github action IPs/domains(?) are blocked by cloudflare, others not. It also looks like the linter just tries to get https://nitrokey.com/ and is blocked, as this is the only nitrokey.com url within the metainfo.xml

Nevertheless, the other warnings are still true