johngmyers: Automated cherry pick of #12043: Use regional STS endpoint

JobBuild
12058
07-28 16:21
pull-kops-bazel-build 1420418975283548160
pull-kops-bazel-test 1420418975325491200
pull-kops-e2e-kubernetes-aws-1-21 1420418975237410816
pull-kops-verify-boilerplate 1420418975405182976
pull-kops-verify-generated 1420418975363239936
pull-kops-verify-gofmt 1420418975447126016
pull-kops-verify-govet 1420418975484874752