Different Build Commands for Build Previews

Currently, it seems that there is no way to set different build commands in Cloudflare Pages for preview deploys and production deploys.

I would like to run certain tests [html-proofer for example] on the production deploy but typically skip those tests for preview deploys [partially due to speed, partially to failing on new pages, etc…] among other things. It would be super useful to have variations of build commands for production and preview.

If anything, providing an environmental variable like IS_PREVIEW on a preview-deploy would be helpful too, since you can just create conditionals on the build command itself.