Explain why devtools in pinned (#77657)

dev/devicelab/lib/tasks/perf_tests.dart

diff --git a/dev/devicelab/lib/tasks/perf_tests.dart b/dev/devicelab/lib/tasks/perf_tests.dart
index fd08db2..b82e4d4 100644
--- a/dev/devicelab/lib/tasks/perf_tests.dart
+++ b/dev/devicelab/lib/tasks/perf_tests.dart
@@ -1409,6 +1409,11 @@
   }
 
   Future<void> _launchDevTools() async {
+    // The version of devtools is pinned. If we pub global activate devtools and an
+    // upstream devtools release breaks our CI, it will manifest on an unrelated
+    // commit, making it more difficult to determine the cause.
+    //
+    // Also, for release branches, all external test dependencies need to be pinned.
     await exec(pubBin, <String>[
       'global',
       'activate',