store: set nodegroup last activity

Signed-off-by: CrazyMax <crazy-max@users.noreply.github.com>

store/nodegroup.go

@@ -2,6 +2,7 @@ package store
 
 import (
 	"fmt"
+	"time"
 
 	"github.com/containerd/containerd/platforms"
 	"github.com/docker/buildx/util/confutil"
@@ -18,7 +19,8 @@ type NodeGroup struct {
 	Dynamic bool
 
 	// skip the following fields from being saved in the store
-	DockerContext bool `json:"-"`
+	DockerContext bool      `json:"-"`
+	LastActivity  time.Time `json:"-"`
 }
 
 type Node struct {