Use shared vars in Buildkite pipilines and explicitly set mac queue

.buildkite/pipeline.yml

@@ -1,29 +1,26 @@
-# Nodes with values to reuse in the pipeline.
-common_params:
-  # Common plugin settings to use with the `plugins` key.
-  - &common_plugins
-    - automattic/a8c-ci-toolkit#3.1.0
-  # Common environment values to use with the `env` key.
-  - &common_env
-    IMAGE_ID: xcode-15.4
-
 # This is the default pipeline – it will build and test the app
+
+env:
+  IMAGE_ID: $IMAGE_ID
+
 steps:
   - label: "🔬 Build and Test"
     command: .buildkite/commands/build-and-test.sh
-    env: *common_env
-    plugins: *common_plugins
+    plugins: [$CI_TOOLKIT_PLUGIN]
+    agents:
+      queue: mac
 
   - label: ":swift: SwiftLint"
     command: run_swiftlint --strict
-    plugins: *common_plugins
+    plugins: [$CI_TOOLKIT_PLUGIN]
     notify:
       - github_commit_status:
           context: "SwiftLint"
     agents:
-      queue: "default"
+      queue: default
 
   - label: "🛠 Verify App Store Target Builds"
     command: .buildkite/commands/build-and-test-app-store.sh
-    env: *common_env
-    plugins: *common_plugins
+    plugins: [$CI_TOOLKIT_PLUGIN]
+    agents:
+      queue: mac

.buildkite/release-builds.yml

@@ -1,16 +1,9 @@
-# Nodes with values to reuse in the pipeline.
-common_params:
-  # Common plugin settings to use with the `plugins` key.
-  - &common_plugins
-    - automattic/a8c-ci-toolkit#3.1.0
-  # Common environment values to use with the `env` key.
-  - &common_env
-    IMAGE_ID: xcode-15.4
-
-# This is the default pipeline – it will build and test the app
 steps:
   - label: "🛠 App Store Upload"
     command: .buildkite/commands/build-and-upload-release.sh
     priority: 1
-    env: *common_env
-    plugins: *common_plugins
+    plugins: [$CI_TOOLKIT_PLUGIN]
+    agents:
+      queue: mac
+    env:
+      IMAGE_ID: $IMAGE_ID

.buildkite/shared-pipeline-vars

@@ -0,0 +1,11 @@
+#!/bin/sh
+
+# This file is `source`'d before calling `buildkite-agent pipeline upload`, and can be used
+# to set up some variables that will be interpolated in the `.yml` pipeline before uploading it.
+
+# The ~> modifier is not currently used, but we check for it just in case
+XCODE_VERSION=$(sed -E -n 's/^(~> )?(.*)/xcode-\2/p' .xcode-version)
+CI_TOOLKIT_PLUGIN_VERSION="3.4.2"
+
+export IMAGE_ID="$XCODE_VERSION"
+export CI_TOOLKIT_PLUGIN="automattic/a8c-ci-toolkit#$CI_TOOLKIT_PLUGIN_VERSION"

.xcode-version

@@ -0,0 +1 @@
+15.4