Set default isolated_server host in try.try_builder()

This is a follow up of https://crrev.com/c/2397055. It fixes failures in 'luci.chromium.try' pool like https://chromium-swarm.appspot.com/task?id=4e89981464fc8310

Bug: 894045, 1126307
Change-Id: I3e80f289c119e237c160a860ba4024f1c80c58dc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2397981Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Commit-Queue: Ye Kuang <yekuang@google.com>
Cr-Commit-Position: refs/heads/master@{#805200}

infra/config/lib/try.star

@@ -242,6 +242,7 @@ def try_builder(
         resultdb_bigquery_exports = [resultdb.export_test_results(
             bq_table = "luci-resultdb.chromium.try_test_results",
         )],
+        isolated_server = "https://isolateserver.appspot.com",
         **kwargs
     )