First attempt at having a macOS build in Buildkite

.buildkite/pipeline.yml

@@ -0,0 +1,13 @@
+agents:
+  queue: mac
+env:
+  IMAGE_ID: $IMAGE_ID
+
+steps:
+  - label: Test code signing fetching
+    command: |
+      npm ci
+      make package-osx
+    plugins:
+      - $CI_TOOLKIT_PLUGIN
+      - $NVM_PLUGIN

.buildkite/shared-pipeline-vars

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+#!/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=$(sed -E -n 's/^(~> )?(.*)/xcode-\2/p' .xcode-version)
+CI_TOOLKIT_PLUGIN_VERSION='3.3.0'
+NVM_PLUGIN_VERSION='0.3.0'
+
+export IMAGE_ID="$XCODE_VERSION"
+export CI_TOOLKIT_PLUGIN="automattic/a8c-ci-toolkit#$CI_TOOLKIT_PLUGIN_VERSION"
+export NVM_PLUGIN="automattic/nvm#$NVM_PLUGIN_VERSION"

.xcode-version

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