ci: Use script file to simplify current working directory management

Various Buildkite commands failed when using Makefile commands as the
current working directory is not persisted with each new line.

.buildkite/pipeline.yml

@@ -31,5 +31,7 @@ steps:
       plugins: *plugins
 
     - label: ':ios: Publish iOS Artifacts'
-      command: make publish-ios-artifacts
+      command: |
+          make build
+          .buildkite/scripts/publish-ios-artifacts.sh
       plugins: *plugins

.buildkite/scripts/publish-ios-artifacts.sh

@@ -0,0 +1,11 @@
+#!/bin/bash -eu
+
+echo "--- :rubygems: Installing Ruby Dependencies"
+pushd ./ios > /dev/null
+install_gems
+
+echo "--- :s3: Uploading iOS artifacts to S3"
+bundle exec fastlane upload_ios_artifacts_to_s3
+
+# Restore the working directory
+popd > /dev/null

Makefile

@@ -46,10 +46,3 @@ build-swift-package: build
 
 test-swift-package: build
 	$(call XCODEBUILD_CMD, test)
-
-publish-ios-artifacts: build
-	echo "--- :rubygems: Installing Ruby Dependencies"
-	cd ./ios && \
-	install_gems && \
-	echo "--- :s3: Uploading iOS artifacts to S3" && \
-	bundle exec fastlane upload_ios_artifacts_to_s3