Post the command to try a pull request on the pull request

A PR gets a ready-to-paste command for every script it touches. An install
script has no entry point of its own, so it is mapped back to the ct/ script
that pulls it in, and a PR touching both halves of one script produces one
command rather than two.

The comment spells out why COMMUNITY_SCRIPTS_URL has to be set: fetching a ct/
script from a branch does not tell the engine where that branch is, so without
it the install script would be looked up in upstream main.

pull_request_target for fork PRs, with nothing from the pull request checked
out or executed -- branch name and file list come from the API, and the comment
is assembled in JavaScript.

Author: MickLesk