pkg/tool/exec: fix stdin processing

Change-Id: I192ac14ae1123fd115c048c44fec18cd37a626f7
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/4600
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>
diff --git a/internal/task/task.go b/internal/task/task.go
index a6e563c..a6a54d2 100644
--- a/internal/task/task.go
+++ b/internal/task/task.go
@@ -27,6 +27,7 @@
 // A Context provides context for running a task.
 type Context struct {
 	Context context.Context
+	Stdin   io.Reader
 	Stdout  io.Writer
 	Stderr  io.Writer
 	Obj     cue.Value