[dart/fuzzer] Reduce number of runs to avoid timeouts

Change-Id: I582dbe87a84511c63aeb6dc0cfa52156d33c015a
Reviewed-on: https://dart-review.googlesource.com/c/83660
Auto-Submit: Alexander Thomas <[email protected]>
Reviewed-by: Aart Bik <[email protected]>
Commit-Queue: Aart Bik <[email protected]>

Author: athomas

tools/bots/test_matrix.json

@@ -1767,7 +1767,7 @@
             "8",
             "--no-show-stats",
             "--repeat",
-            "125"
+            "100"
           ],
           "shards": 10,
           "fileset": "fuzzer"