sortfields does not sort some struct fields in cue eval when there are cyclic references

[mvdan]

Note that this issue is related to #3621, as evalv3 runs into cyclic reference errors with the input below, where evalv2 does not. It's possible that that's causing CUE_DEBUG=sortfields to not work properly. However, if cue eval is producing some CUE output, I would expect all fields to be sorted, even if the output is not complete or contains errors such as missing references.

env CUE_DEBUG=sortfields

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

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

-- input.cue --
package p

resource: {
	outer: [Name=string]: name: Name
	if true {
		inner: inner2: {
			name:           "Employees"
			description:    "All employees"
			privacy:        "secret"
			create_default: false
		}
		outer: outer2: {
			bar: (inner.inner2.name)
		}
	}
}

As of 0e99c20:

> env CUE_DEBUG=sortfields
# With the old evaluator. (0.010s)
> env CUE_EXPERIMENT=evalv3=0
> exec cue eval
[stdout]
resource: {
    inner: {
        inner2: {
            create_default: false
            description:    "All employees"
            name:           "Employees"
            privacy:        "secret"
        }
    }
    outer: {
        outer2: {
            bar:  "Employees"
            name: "outer2"
        }
    }
}
# With the new evaluator. (0.030s)
> env CUE_EXPERIMENT=evalv3=1
> exec cue eval
[stdout]
resource: {
    outer: {
        [Name=string]: {
            name: Name
        }
    }
    if true {
        inner: {
            inner2: {
                name:           "Employees"
                description:    "All employees"
                privacy:        "secret"
                create_default: false
            }
        }
        outer: {
            outer2: {
                bar: inner.inner2.name
            }
        }
    }
}

The output is slightly different in general, which is fine - cue eval does not follow a strict behavior so there are differences between v2 and v3. However, inner2's fields are properly sorted on evalv2, wheras they are not sorted on evalv3.