Fix Bitrise's ciRunURL underlying env var

[rogerluan]

Hey 👋

During some debugging process I realized that this env var is probably wrong, based on what it means and what other similar CI files are using as env var values 😊 So here's the fix!

I also added a handy link at the bottom of the file, copying GitLabCI.ts and Travis.ts 📜

EDIT: Resolves #1114

[orta]

Is it possible for people to have older versions of bitrise ( and thus need the old version around?)

[rogerluan]

Hey @orta ! Just like in my other PR, both of these env vars (old and new) are also global env vars and not produced by a step, so they should be the same for everyone.

But in this case I don't even think this was ever correct - as the old env var BITRISE_PULL_REQUEST represents just a number (e.g. 1113), not anything else (like the PR link or anything). This might've been a typo or copy'n'paste oversight. The underlying value of this env var was expected to be a number because that's even in the isPR() function a few lines above 😄

So I'd say the answer is no! :)