Enable luci_flags for faster builds (#164069)

Experiment enable turning on faster builds.

1. Moves collecting of builds till after we setup the orchestrator bot
2. Downloads drone artifacts in parallel.

See: https://flutter-review.googlesource.com/c/recipes/+/63800

Author: jtmcdole

engine/src/flutter/ci/builders/mac_host_engine.json

@@ -10,6 +10,10 @@
         "Tests to run on mac hosts should go in one of the other mac_ build ",
         "definition files."
     ],
+    "luci_flags": {
+      "delay_collect_builds": true,
+      "parallel_download_builds": true
+    },
     "builds": [
         {
             "drone_dimensions": [

engine/src/flutter/ci/builders/mac_ios_engine.json

@@ -7,6 +7,10 @@
         "Tests to run on mac hosts should go in one of the other mac_ build ",
         "definition files."
     ],
+    "luci_flags": {
+      "delay_collect_builds": true,
+      "parallel_download_builds": true
+    },
     "builds": [
         {
             "drone_dimensions": [