tools/flow: support task results when defined via definitions

This is not a very common use case, but it should not panic,
and it should work just like it does when tasks do not have results.

Fixes #2633.

Signed-off-by: Daniel Martí <[email protected]>
Change-Id: I45a939d2e55ae756ee9a3a809722e86637a2084d
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1224695
TryBot-Result: CUEcueckoo <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>
Reviewed-by: Roger Peppe <[email protected]>

Author: mvdan

cmd/cue/cmd/testdata/script/cmd_hidden.txtar

@@ -1,5 +1,14 @@
-! exec cue cmd test
-stderr 'panic: unexpected label type'
+exec cue cmd test
+cmp stdout stdout.want
+
+-- stdout.want --
+hello world
+
+hello world
+
+hello world
+
+hello world
 
 -- read.txt --
 hello world

tools/flow/run.go

@@ -211,7 +211,7 @@ func (c *Controller) updateTaskResults(t *Task) bool {
 	expr := t.update
 	for _, label := range slices.Backward(t.labels) {
 		switch label.Typ() {
-		case adt.StringLabel, adt.HiddenLabel:
+		case adt.StringLabel, adt.HiddenLabel, adt.DefinitionLabel, adt.HiddenDefinitionLabel, adt.LetLabel:
 			expr = &adt.StructLit{
 				Decls: []adt.Decl{
 					&adt.Field{