🦉 Updates from OwlBot post-processor

See https://github.com/googleapis/repo-automation-bots/blob/main/packages/owl-bot/README.md

Author: gcf-owl-bot[bot]

.kokoro/presubmit/presubmit.cfg

@@ -1,7 +1,7 @@
 # Format: //devtools/kokoro/config/proto/build.proto
 
-# Only run a subset of all nox sessions
+# Disable system tests.
 env_vars: {
-    key: "NOX_SESSION"
-    value: "unit-3.8 unit-3.12 cover docs docfx"
+    key: "RUN_SYSTEM_TESTS"
+    value: "false"
 }