Make engine.#Scratch a specialization of #FS, rather than a task

Signed-off-by: Joel Longtine <joel@dagger.io>

plan/task/scratch.go

@@ -1,24 +0,0 @@
-package task
-
-import (
-	"context"
-
-	"go.dagger.io/dagger/compiler"
-	"go.dagger.io/dagger/plancontext"
-	"go.dagger.io/dagger/solver"
-)
-
-func init() {
-	Register("Scratch", func() Task { return &scratchTask{} })
-}
-
-type scratchTask struct {
-}
-
-func (t *scratchTask) Run(ctx context.Context, pctx *plancontext.Context, s solver.Solver, v *compiler.Value) (*compiler.Value, error) {
-	fs := pctx.FS.New(nil)
-
-	return compiler.NewValue().FillFields(map[string]interface{}{
-		"output": fs.MarshalCUE(),
-	})
-}