Convert the bool `beta` flag to string for Buildkite

[mokagio]

This should address the failure seen running the new buildkite_pipeline_upload in CI:

/opt/ci/builds/builder/automattic/simplenote-ios/vendor/bundle/ruby/3.2.0/gems/fastlane-2.225.0/fastlane/lib/fastlane/helper/sh_helper.rb:118:in `block in shell_command_from_args': [!] undefined method `shellescape' for true:TrueClass (NoMethodError)
      command = args.shift.map { |k, v| "#{k}=#{v.shellescape}" }.join("") + " "

https://buildkite.com/automattic/simplenote-ios/builds/1184#0192d797-332d-4bed-990a-0101000f2f56

[dangermattic]

	1 Warning
:warning:	This PR is assigned to the milestone 4.55 ❄️. The due date for this milestone has already passed. Please assign it to a milestone with a later deadline or check whether the release for this milestone has already been finished.

Generated by :no_entry_sign: Danger

[mokagio]

Bypassing CI checks because this change does not run in the standard CI pipeline. We'll validate it next by retrying the complete code freeze pipeline.

[wpmobilebot]

📲 You can test the changes from this Pull Request in Simplenote Prototype Build by scanning the QR code below to install the corresponding build.

	App Name	Simplenote Prototype Build
	Build Number	pr1669-5539e0f-0192d79b-e603-445e-8f2a-0e59b2340e4f
	Version	4.55
	Bundle ID	com.codality.NotationalFlow.Alpha
	Commit	5539e0fd62906ba874328f8471a946e41e23e2ca
	App Center Build	Simplenote - Installable Builds #394

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