nixosTest: Fix infinite recursion involving hasContext testScript when useNixStoreImage = true
nixosTest: Fix infinite recursion involving hasContext testScript when useNixStoreImage = true

The involved test was nixosTests.nextcloud.basic21.

It has a testScript that is strict in nodes.nextcloud.config.system.build.vm,
in assertions about imagemagick being in the system closure.

The recursion was introduced in 329a4461a7a1898b3aad13968a232a054422c4fb from
https://github.com/NixOS/nixpkgs/pull/140792

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