Run more builds faster (#164125)

The following builds all download drone assets and run generators, and
thus could benefit from the flags.

- Linux linux_android_aot_engine
- Linux linux_android_debug_engine
- Linux linux_fuchsia
- Linux linux_host_engine

See #164069 for other builds

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

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

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

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

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

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

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

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