fix `--exit` flag in dev/devicelab/bin/run.dart (#134162)

Fixes #134154 

This PR also changes the default value of the `--exit` flag from `true` to `false`. Effectively, this is not a change in behavior since `--exit` didn't previously work.

dev/devicelab/bin/run.dart

@@ -293,7 +293,6 @@ ArgParser createArgParser(List<String> taskNames) {
     )
     ..addFlag(
       'exit',
-      defaultsTo: true,
       help: 'Exit on the first test failure. Currently flakes are intentionally (though '
             'incorrectly) not considered to be failures.',
     )

dev/devicelab/lib/framework/runner.dart

@@ -47,8 +47,8 @@ Future<void> runTasks(
 }) async {
   for (final String taskName in taskNames) {
     TaskResult result = TaskResult.success(null);
-    int retry = 0;
-    while (retry <= Cocoon.retryNumber) {
+    int failureCount = 0;
+    while (failureCount <= Cocoon.retryNumber) {
       result = await rerunTask(
         taskName,
         deviceId: deviceId,
@@ -66,11 +66,14 @@ Future<void> runTasks(
       );
 
       if (!result.succeeded) {
-        retry += 1;
+        failureCount += 1;
+        if (exitOnFirstTestFailure) {
+          break;
+        }
       } else {
         section('Flaky status for "$taskName"');
-        if (retry > 0) {
-          print('Total ${retry+1} executions: $retry failures and 1 false positive.');
+        if (failureCount > 0) {
+          print('Total ${failureCount+1} executions: $failureCount failures and 1 false positive.');
           print('flaky: true');
           // TODO(ianh): stop ignoring this failure. We should set exitCode=1, and quit
           // if exitOnFirstTestFailure is true.
@@ -84,7 +87,7 @@ Future<void> runTasks(
 
     if (!result.succeeded) {
       section('Flaky status for "$taskName"');
-      print('Consistently failed across all $retry executions.');
+      print('Consistently failed across all $failureCount executions.');
       print('flaky: false');
       exitCode = 1;
       if (exitOnFirstTestFailure) {