evalv3 "field not allowed" regression involving embeddings and selectors

[mvdan]

# evalv2
env CUE_EXPERIMENT=evalv3=0
exec cue export

# evalv3
env CUE_EXPERIMENT=evalv3=1
exec cue export

-- input.cue --
package p

#Schema: {
	msg: "hello"
	unused: {
		{embed: embedded: "foo"}.embed
	} & {
		embedded: string
		extra: "never printed"
	}
}

// Remove `& {}` below to fix evalv3.
out: {result: (#Schema & {}).msg}.result

As of 447e98d:

# evalv2 (0.010s)
> env CUE_EXPERIMENT=evalv3=0
> exec cue export
[stdout]
{
    "out": "hello"
}
# evalv3 (0.028s)
> env CUE_EXPERIMENT=evalv3=1
> exec cue export
[stderr]
unused.extra: field not allowed:
    ./input.cue:14:16
    ./input.cue:9:3
[exit status 1]
FAIL: repro-evalv3.txtar:7: unexpected command failure

Reduced from @nyarly's project in Unity.