syz-gce: allow to not recreate image, rebuild syzkaller
syz-gce: allow to not recreate image, rebuild syzkaller

Merely for testing/development when one needs to restart it frequently.

hyz0906/syzkaller,xairy/syzkaller,hyz0906/syzkaller,sploving/syzkaller,xairy/syzkaller,xairy/syzkaller,google/syzkaller,google/syzkaller,google/syzkaller,dotweiba/syzkaller,sploving/syzkaller,zoulasc/syzkaller,dotweiba/syzkaller,dotweiba/syzkaller,hyz0906/syzkaller,xairy/syzkaller,google/syzkaller,xairy/syzkaller,sploving/syzkaller,google/syzkaller,hyz0906/syzkaller,zoulasc/syzkaller,google/syzkaller,google/syzkaller,zoulasc/syzkaller,dotweiba/syzkaller,hyz0906/syzkaller,xairy/syzkaller,sploving/syzkaller,zoulasc/syzkaller,xairy/syzkaller