Fix stray let in I18n preventing uglify (into main)

[colinrotherham]

This is a copy of #2868 with main as the base

I've also included a required bug fix:

Fix for a YAML to JSON “write after end” race condition
The gulp copy-files stream can only write one JSON file fixtures.json or macro-options.json as shown

Original description

This PR fixes a tiny let → var which caused a fairly obscure GulpUglifyError

[18:55:27] GulpUglifyError: unable to minify JavaScript
Caused by: SyntaxError: Unexpected token: name «placeholderValue», expected: punc «;»
File: /Users/colin/Sites/GDS/govuk-frontend/src/govuk/all.mjs
Line: 524
Col: 10

To catch this next time I've added the other npm build:* scripts to the GitHub workflow:

- name: Build
  run: |
    npm run build:assets
    npm run build:package
    npm run build:dist

[colinrotherham]

Changes to this file have been brought over from #2862