Make SHORTNAME detection on GitHub Actions much simpler

[foolip]

The complicated thing is still used locally and on Travis.

[foolip]

Here I also organized things a bit into constants, environment variables and local variables.

[annevk]

So we basically have this because of compat/console/streams not using their shortname as their .bs name, right? @domenic any thoughts on renaming them instead?

[domenic]

Renaming is quite disruptive and loses the entire file history in the GitHub UI. I think adding a single line in the build script is a very small price to pay.

(I'm not really sure why we'd expand that single line into 5 lines as this PR does, though...)

[annevk]

Well, you wouldn't lose history for the links we advocate, e.g., https://github.com/whatwg/compat/commits, and I think blame isn't impacted either, but that's fair.

[foolip]

I'm not really sure why we'd expand that single line into 5 lines as this PR does, though...

Things work without this change, but interpreting that sed command is very hard. This is a follow-up to #325 to make it more obvious why it's like that (in the future: for local builds only). But if this just looks more convoluted, I could either:

• abandon this
• just use the directory name in the else branch

[annevk]

I would abandon, but explain the complexity in a comment.

[domenic]

Given that the sed command still exists, it doesn't feel like a win to just add a second path where the sed command is bypassed. Now one has to understand complicated thing + simple thing, which seems slightly worse than having to understand complicated thing by itself.

[foolip]

I've sent #327 instead.