Fix export cache issue

Resolve #1551 and #1020.
We are never returning the result of solved operations so Buildkit could not
cache the layer.
This commit implements a simple system to forward operations' result to the
main build to cache it.

Signed-off-by: Vasek - Tom C <tom.chauveau@epitech.eu>

plan/task/nop.go

@@ -15,6 +15,6 @@ func init() {
 type nopTask struct {
 }
 
-func (t *nopTask) Run(ctx context.Context, pctx *plancontext.Context, s solver.Solver, v *compiler.Value) (*compiler.Value, error) {
+func (t *nopTask) Run(_ context.Context, _ *plancontext.Context, _ *solver.Solver, v *compiler.Value) (*compiler.Value, error) {
 	return v, nil
 }