In structured attrs derivations, __json is used to serialize the `env`
attribute into a JSON object.
If the derivation enables structured attributes AND specify its own
`__json`, one of the field will take precedence (the reader can have fun
by guessing which one).
To reduce underspecification, we disallow `__json` in structured
attributes derivations.
Fixes#380.
Change-Id: I51c3b2af1ff9449471ba81d4c72df6a7a263eef7
Signed-off-by: Raito Bezarius <raito@lix.systems>
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