Fix assertion that config.nixpkgs cannot be set if useGlobalPkgs is enabled

[justDeeevin]

Description

When useGlobalPkgs is enabled, setting nixpkgs.config and nixpkgs.overlays in the home config has no effect. There is an assertion in place intended to throw an error if the user tries to set these options in this scenario, but it uses an OR instead of an AND, allowing for one of them to be set. This PR replaces this OR for an AND.

Fixes #6079.

Checklist

• Change is backwards compatible.

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Maintainer CC

@thiagokokada

[justDeeevin]

Been a minute... what's left to get this merged?

[Adda0]

LGTM. I do not think there is anything left to do. The PR just needs to wait for someone to merge it.