[CI] Bump disk size to 200GB.

If the disk size is 100GB, we run out of disk space and k8s starts evicting
pods, which show up as somewhat obtuse failures in Github Actions.

premerge/main.tf

@@ -91,6 +91,7 @@ resource "google_container_node_pool" "llvm_premerge_linux" {
     labels = {
       "premerge-platform" : "linux"
     }
+    disk_size_gb = 200
   }
 }