Revert "[CI] Disable autoscaling on Linux node pool"

This reverts commit cff6f90.

This was reasonably successful. We have not seen any new failures, which helps
confirm our hypothesis that this is related to konnectivity-agent. We do not
want to launch without autoscaling, so the plan is to disable the
kubernetes executor mode at launch.

premerge/main.tf

@@ -75,7 +75,11 @@ resource "google_container_node_pool" "llvm_premerge_linux" {
   location           = "us-central1-a"
   cluster            = google_container_cluster.llvm_premerge.name
   initial_node_count = 0
-  node_count         = 4
+
+  autoscaling {
+    total_min_node_count = 0
+    total_max_node_count = 4
+  }
 
   node_config {
     machine_type = "n2-standard-64"