evalv3: incomplete value regression involving builtin calls and regular expressions

[mvdan]

# With the old evaluator.
env CUE_EXPERIMENT=evalv3=0
exec cue vet -c

# With the new evaluator.
env CUE_EXPERIMENT=evalv3=1
exec cue vet -c

-- input.cue --
package p

import "list"

_environ: FOO: "bar"
env: null | [...=~"="]
env: list.Concat([[for k, v in _environ { "\(k)=\(v)" }]])

As of 9e85cd7:

# With the old evaluator. (0.010s)
> env CUE_EXPERIMENT=evalv3=0
> exec cue vet -c
# With the new evaluator. (0.029s)
> env CUE_EXPERIMENT=evalv3=1
> exec cue vet -c
[stderr]
env.0: incomplete value =~"=":
    ./input.cue:6:17
[exit status 1]