Increase operator-test Cloud Build disk size

[bstoll]

The operator-test Cloud Build workflow failed because the image did not have enough disk space to bring everything up:

I0304 18:23:34.995057    2165 deploy.go:768] Loading "us-west1-docker.pkg.dev/gep-kne/kne/kubebuilder/kube-rbac-proxy:v0.11.0" as "gcr.io/kubebuilder/kube-rbac-proxy:v0.11.0"
W0304 18:23:39.694506    2165 run.go:29] (kind): Image: "gcr.io/kubebuilder/kube-rbac-proxy:v0.11.0" with ID "sha256:a4f6c39b17f2ba68366c6d4e139ed18ede2ad9f3f9d5bf7477bdcbef73b43e54" not yet present on node "kne-control-plane", loading...
I0304 18:23:41.341870    2165 deploy.go:768] Loading "us-west1-docker.pkg.dev/gep-kne/kne/kubebuilder/kube-rbac-proxy:v0.12.0" as "gcr.io/kubebuilder/kube-rbac-proxy:v0.12.0"
W0304 18:23:45.196181    2165 run.go:29] (kind): Image: "gcr.io/kubebuilder/kube-rbac-proxy:v0.12.0" with ID "sha256:a5a3afc587c86f484e20e279dbaa2b08e2a4c8957e4424b8d5334006c22c6fbe" not yet present on node "kne-control-plane", loading...
I0304 18:23:47.177822    2165 deploy.go:768] Loading "us-west1-docker.pkg.dev/gep-kne/kne/kubebuilder/kube-rbac-proxy:v0.13.1" as "gcr.io/kubebuilder/kube-rbac-proxy:v0.13.1"
W0304 18:23:53.227298    2165 run.go:29] (kind): Image: "gcr.io/kubebuilder/kube-rbac-proxy:v0.13.1" with ID "sha256:18f12fc3a0085bbc303cfbd2ad1151127c1c959c4641e36027547b6b66eb4c92" not yet present on node "kne-control-plane", loading...
W0304 18:23:53.229647    2165 run.go:29] (kind): ERROR: failed to create tempdir: mkdir /tmp/images-tar58482254: no space left on device

I verified this change resolves the issue by manually invoking Cloud Build with this change here. The preemptable flag is removed because we don't want GCP killing our tests - this can happen sometimes.