Move gclient var property to ci.yaml (flutter#37351)

This property was previously hardcoded in the recipe.

Bug: flutter/flutter#114656

Author: godofredoc

.ci.yaml

@@ -268,6 +268,8 @@ targets:
       add_recipes_cq: "true"
       cores: "32"
       gcs_goldens_bucket: flutter_logs
+      gclient_custom_vars: >-
+        {"download_emsdk": True}
       dependencies: >-
         [
           {"dependency": "goldctl", "version": "git_revision:3a77d0b12c697a840ca0c7705208e8622dc94603"}
@@ -407,6 +409,8 @@ targets:
     properties:
       add_recipes_cq: "true"
       gcs_goldens_bucket: flutter_logs
+      gclient_custom_vars: >-
+        {"download_emsdk": True}
       dependencies: >-
         [
           {"dependency": "goldctl", "version": "git_revision:3a77d0b12c697a840ca0c7705208e8622dc94603"}
@@ -481,6 +485,8 @@ targets:
   - name: Windows Web Engine
     recipe: engine/web_engine
     properties:
+      gclient_custom_vars: >-
+        {"download_emsdk": true}
       gcs_goldens_bucket: flutter_logs
     timeout: 60
     runIf: