NLS can get mixed up when `stable` overwrites `insider` metadata

[VSCodeTriageBot]

Here is how that can happen:

• our NLS metadata is accessed from https://main.vscode-cdn.net/nlsmetadata/${info.commit}/nls.messages.js
• when we prepare for a release it can happen that we build stable and insider from the same commit
• each build will produce a different order of NLS messages which is because we process messages from gulp and that seems to have a somewhat random order per execution

Ideally we would ensure stable order of NLS message processing, maybe by sorting the file names first.