evalv3 regression: key value of dynamic field must be concrete, found _|_

[mvdan]

# With the old evaluator.
env CUE_EXPERIMENT=evalv3=0
exec cue export

# With the new evaluator.
env CUE_EXPERIMENT=evalv3=1
exec cue export

-- input.cue --
package p

orgs: org1: {
	repos: repo1: {}
}

botUser: "bot1"

orgs: [_]: config={
	for repo_name, _ in config.repos
	let orgBotUser = botUser {
		collaborators: (repo_name): (orgBotUser): true
	}
}

As of c479844:

# With the old evaluator. (0.022s)
> env CUE_EXPERIMENT=evalv3=0
> exec cue export
[stdout]
{
    "orgs": {
        "org1": {
            "repos": {
                "repo1": {}
            },
            "collaborators": {
                "repo1": {
                    "bot1": true
                }
            }
        }
    },
    "botUser": "bot1"
}
# With the new evaluator. (0.048s)
> env CUE_EXPERIMENT=evalv3=1
> exec cue export
[stderr]
orgs.org1.collaborators.repo1: key value of dynamic field must be concrete, found _|_(orgs.org1.collaborators.repo1: cycle with field: orgBotUser):
    ./input.cue:12:32
[exit status 1]
FAIL: repro-evalv3.txtar:7: unexpected command failure

Reduced from our own infra repo.

[mpvl]

Somewhat reduced:

a: b: {}
a: [_]: {
	if true let x=str {
		(x): true
	}
}
str: "bot1"