Force a sync to avoid log truncation
Force a sync to avoid log truncation

Compare the following files:
- https://storage.googleapis.com/kubernetes-jenkins/logs/post-node-driver-registrar-push-images/1252806915545632768/artifacts/build.log
- https://console.cloud.google.com/cloud-build/builds/a547448a-39e3-4154-9618-5617f0b34319;step=0?project=k8s-staging-csi

You can see that the first url has some text cut off at the end. File
created by `os.Create` is unbuffered, so there is nothing to flush.
Let's try `Sync` to see if that helps

Signed-off-by: Davanum Srinivas <36003aec7ba325a16727bc9a176c12d2776a8ce4@gmail.com>

BenTheElder/test-infra,dims/test-infra,ixdy/kubernetes-test-infra,ixdy/kubernetes-test-infra,michelle192837/test-infra,monopole/test-infra,brahmaroutu/test-infra,monopole/test-infra,cblecker/test-infra,pwittrock/test-infra,dims/test-infra,cjwagner/test-infra,michelle192837/test-infra,dims/test-infra,cjwagner/test-infra,monopole/test-infra,kubernetes/test-infra,dims/test-infra,michelle192837/test-infra,brahmaroutu/test-infra,monopole/test-infra,cblecker/test-infra,cblecker/test-infra,jessfraz/test-infra,brahmaroutu/test-infra,pwittrock/test-infra,dims/test-infra,fejta/test-infra,fejta/test-infra,kubernetes/test-infra,fejta/test-infra,cblecker/test-infra,fejta/test-infra,kubernetes/test-infra,kubernetes/test-infra,cjwagner/test-infra,pwittrock/test-infra,cjwagner/test-infra,BenTheElder/test-infra,monopole/test-infra,dims/test-infra,cblecker/test-infra,fejta/test-infra,pwittrock/test-infra,BenTheElder/test-infra,cjwagner/test-infra,jessfraz/test-infra,pwittrock/test-infra,ixdy/kubernetes-test-infra,brahmaroutu/test-infra,cblecker/test-infra,kubernetes/test-infra,kubernetes/test-infra,brahmaroutu/test-infra,brahmaroutu/test-infra,michelle192837/test-infra,fejta/test-infra,jessfraz/test-infra,michelle192837/test-infra,michelle192837/test-infra,ixdy/kubernetes-test-infra,ixdy/kubernetes-test-infra,BenTheElder/test-infra,jessfraz/test-infra,BenTheElder/test-infra,jessfraz/test-infra,cjwagner/test-infra,monopole/test-infra,jessfraz/test-infra,BenTheElder/test-infra