Allow for nil buildkit References

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

plan/task/scratch.go

@@ -3,7 +3,6 @@ package task
 import (
 	"context"
 
-	"github.com/moby/buildkit/client/llb"
 	"go.dagger.io/dagger/compiler"
 	"go.dagger.io/dagger/plancontext"
 	"go.dagger.io/dagger/solver"
@@ -17,13 +16,7 @@ type scratchTask struct {
 }
 
 func (t *scratchTask) Run(ctx context.Context, pctx *plancontext.Context, s solver.Solver, v *compiler.Value) (*compiler.Value, error) {
-	st := llb.Scratch()
-	result, err := s.Solve(ctx, st, pctx.Platform.Get())
-	if err != nil {
-		return nil, err
-	}
-
-	fs := pctx.FS.New(result)
+	fs := pctx.FS.New(nil)
 
 	return compiler.NewValue().FillFields(map[string]interface{}{
 		"output": fs.MarshalCUE(),