[dartfuzz] Reduce duration so it finishes before the nightly builders start.

Change-Id: I70f93c97cc89c23c900df951920e21c7f15d6cba
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/450944
Commit-Queue: Ryan Macnak <[email protected]>
Reviewed-by: Alexander Aprelev <[email protected]>

Author: rmacnak-google

runtime/tools/dartfuzz/flag_fuzzer.dart

@@ -135,7 +135,7 @@ List<String> someGenSnapshotFlags() {
 
 Stopwatch stopwatch = new Stopwatch();
 
-const overallTimeout = Duration(minutes: 45);
+const overallTimeout = Duration(minutes: 30);
 Duration get remainingTimeout => overallTimeout - stopwatch.elapsed;
 
 // LUCI will kill recipe steps if they go 1200 seconds without any output.

tools/bots/test_matrix.json

@@ -3513,7 +3513,7 @@
             "8",
             "--no-show-stats",
             "--time",
-            "2700"
+            "1800"
           ],
           "shards": 50,
           "fileset": "fuzzer"