feat: change coveo key based on build environment

[JTorreG]

Proposed changes

Logic added to create the coveo.js file with a different coveo key (prod and test) depending on the Hugo build environment.

Production uses the "production" key
Staging uses the "staging" key
Every other environment uses the "development" key

Tested locally, -e production -e development and -e staging , developer console shows a different key in the minified coveo.js

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING document
• If applicable, I have added tests that prove my fix is effective or that my feature works
• If applicable, I have checked that any relevant tests pass after adding my changes
• I have updated any relevant documentation (README.md and CHANGELOG.md)

[jputrino]

The only reservation I have with this approach is that we would need to make any code change to 3 different files. Is there a less duplicative way to solve the problem?

[jputrino]

Nice work! LGTM