parameterize channel

cloud_build/flutter_agy_docker/orchestrator.yaml

diff --git a/cloud_build/flutter_agy_docker/orchestrator.yaml b/cloud_build/flutter_agy_docker/orchestrator.yaml
index 2d6c92b..92ee10e 100644
--- a/cloud_build/flutter_agy_docker/orchestrator.yaml
+++ b/cloud_build/flutter_agy_docker/orchestrator.yaml
@@ -11,22 +11,23 @@
       - |
         set -euo pipefail
 
-        echo "Fetching Flutter release data..."
+        echo "Fetching Flutter release data for channel: $_CHANNEL..."
 
-        # 1. Fetch latest stable version
+        # 1. Fetch latest version for the specified channel (stable or beta)
         # The -f flag on curl ensures it fails on HTTP errors.
+        # We pass the Cloud Build substitution $_CHANNEL as an argument to jq.
         FLUTTER_VERSION=$$(curl -sf https://storage.googleapis.com/flutter_infra_release/releases/releases_linux.json | \
-          jq -r '.current_release.stable as $$stable_hash | .releases[] | select(.hash == $$stable_hash) | .version')
+          jq -r --arg channel "$_CHANNEL" '.current_release[$$channel] as $$hash | .releases[] | select(.hash == $$hash) | .version')
 
         # Validate that the version was successfully parsed.
         # -z "$$FLUTTER_VERSION" checks if the variable is an empty string (e.g., if jq failed entirely).
         # "$$FLUTTER_VERSION" == "null" checks if jq succeeded but couldn't find the requested data in the JSON.
         if [ -z "$$FLUTTER_VERSION" ] || [ "$$FLUTTER_VERSION" == "null" ]; then
-          echo "Error: Failed to parse stable Flutter version from JSON."
+          echo "Error: Failed to parse $_CHANNEL Flutter version from JSON."
           exit 1
         fi
 
-        echo "Found Flutter version: $$FLUTTER_VERSION"
+        echo "Found $_CHANNEL Flutter version: $$FLUTTER_VERSION"
 
         # 2. Trigger the target build with the version as a substitution
         # Note: We use --region=global as found in your trigger description
@@ -34,3 +35,6 @@
             --region=global \
             --branch=main \
             --substitutions=_FLUTTER_VERSION="$$FLUTTER_VERSION"
+
+substitutions:
+  _CHANNEL: "stable"