How do I serve a single html page when the main branch builds with Jekyll?

I have a Jekyll blog than runs on the apex domain.

I want to serve a single html page (non Jekyll) on a branch sub domain. To do so, I have created an empty branch with the single html file. However, when I push it, the deploy fails.

10:00:37 PM: $ jekyll build
10:00:37 PM: bash: jekyll: command not found
10:00:37 PM: ​
10:00:37 PM: ────────────────────────────────────────────────────────────────
10:00:37 PM:   "build.command" failed                                        

How do I deploy the branch?

Change the build command for that branch to "echo Hi!" or something similar: Introducing Deploy Contexts in Netlify

Thanks. Yes, I did almost exactly that. I changed the build command to echo 'done'.