A script essentially runs `v2-install cabal-install:exe:cabal`,
but does some simple setup, so the result is less luckily
to be influences by environment.

Also improve bootstrap to do out-of-tree builds.
- Use http, https tricky in some environments

https://github.com/actions/virtual-environments/blob/releases/ubuntu18/20200604/images/linux/Ubuntu1804-README.md

- msys2 should be installed
- there is cabal and GHC, but we want to be sure about
  versions, so we reinstall.
- Reinstalling GHC doesnt' "just work", so we uninstall it first.
  This takes time, but makes CI work.

- [ci docs] doesn't work.
  We require CI to pass for non-admins to merge, so `[ci docs]` is
  more harmful. Also GitHub actions are reasonably fast,
  so overall it's ok to let it make builds

- Mention `changelog.d` directory

Also add validate-8.10.1 make rule

No "manual" string concatenation,
and the template arguments are typed.

Also split workflows into parts, helps with restarting.
https://github.community/t5/GitHub-Actions/re-run-jobs/m-p/30476#M440

Co-Authored-By: Mikhail Glushenkov <mikhail.glushenkov@gmail.com>

Make a new bug report template.

I noticed that we need to ask whether it's `v1-` or `v2-` build. Maybe a template will help remove single roundtrip.

[ci skip]

[ci skip]

[ci skip]

Shamelessly cribbed from Stack.

[ci skip]