store: set nodegroup last activity

[crazy-max]

adds last activity property that will be updated on build or when a node is updated.

$ docker buildx inspect
Name:          builder2
Driver:        docker-container
Last Activity: 2022-11-30 13:56:51 +0100 CET

Nodes:
Name:           builder20
Endpoint:       unix:///var/run/docker.sock
Driver Options: env.BUILDKIT_STEP_LOG_MAX_SPEED="10485760" env.JAEGER_TRACE="localhost:6831" image="moby/buildkit:master" network="host" env.BUILDKIT_STEP_LOG_MAX_SIZE="10485760"
Status:         running
Flags:          --debug --allow-insecure-entitlement security.insecure --allow-insecure-entitlement network.host
Buildkit:       7615120
Platforms:      linux/amd64, linux/amd64/v2, linux/amd64/v3, linux/arm64, linux/riscv64, linux/ppc64le, linux/s390x, linux/386, linux/mips64le, linux/mips64, linux/arm/v7, linux/arm/v6