[v4.y] CI: don't abort other builds when one fails error

- Helps confirm suspected OS-specific failures.
- Normally when one build fails, most other builds already started and
  some almost complete `bundle install` / started tests.
  So it's not a big "waste" expensive to let them finish, arguably it's
  actually a waste to abort them! (sunken cost fallacy? :-)
- In case rubocop complains, while I do consider it a merge blocker,
  it's better contributor (and maintainer) experience to also see test results.

.github/workflows/actions.yml

@@ -10,6 +10,7 @@ on:
     - '**'
 jobs:
   build:
+    continue-on-error: true
     runs-on: ${{ matrix.os_and_command.os }}
     strategy:
       matrix: