Remove hard coded JEKYLL_ENV var from Jekyll build

[btylerburton]

For staging / demo builds, it is desirable to build Jekyll sites with a customizable JEKYLL_ENV, as certain themes use the env flag for conditional templating logic.

Notes

• Slack convo: https://gsa-tts.slack.com/archives/C04823NMA/p1718994240713269
• Relevant Pages build code:

  pages-build-container/src/steps/build.py

  Line 453 in 0dfbcfe

  env['JEKYLL_ENV'] = 'production'

• Example of how a remote theme we work with is using that flag: https://github.com/open-sdg/open-sdg/blob/2.4.0-dev/_includes/components/indicator/metadata-tabs.html#L29

Acceptance Criteria

• Should only apply to staging / demo builds of a site
• Determine next steps and acceptance criteria