Deploy with GTM_ACCOUNT_ID and move GTM to Top

[ChristopherChudzicki]

This is not a real PR. It exists because #1201 is hard to test locally (requires https + http2).

This PR:

• sets GTM_ACCOUNT_ID for PR deploys to netlify.
• AND moves the GTM script to top of head (the change in #1201)

See https://ocw-hugo-themes-course-v2-pr-1200--ocw-next.netlify.app/

[github-actions[bot]]

Netlify Deployments:
www: https://ocw-hugo-themes-www-pr-1200--ocw-next.netlify.app/
Course v2: https://ocw-hugo-themes-course-v2-pr-1200--ocw-next.netlify.app/