nexusmods-app: init at 0.4.1

Closes #270358.

Name recommended by @erri120
<#270372 (comment)>.

Replaces the `7zz` binary executable included with the upstream project
for safety and reproducibility.

Requires an `enableUnfree = true` override to support RAR format mods.

Disables tests marked as requiring networking (actually requiring Nexus
Mods API key according to
<Nexus-Mods/NexusMods.App#1222 (comment)>),
and one other tests which requires networking.

Co-Authored-By: Matej Cotman <[email protected]>
Co-authored-by: éclairevoyant <[email protected]>
Co-authored-by: Sandro <[email protected]>