nixos/backups/restic: handle cases when both dynamicFileFrom and paths are set

Also, add a test to verify that it works.

This change also removes the part of custom package test that verifies
that the correct paths are provided. This is already tested by restore
tests.

Before this change, setting both paths and dynamicFileFrom would cause
paths to be silently ignored. Making that actually apply the obvious
interpretation seems to me to be strictly better than prohibiting the
two from being set at the same time.

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