Fix CI: don't fail fast, and really allow failure on emacs-snapshot

[bmag]

Before, any failure, even against emacs-snapshot, would cancel all remaining jobs and mark the test ("workflow") as failed. This PR it partially.

Sometimes we need to compare between Emacs versions. With fail-fast set to true (the default), failure in one Emacs version cancels the remaining jobs for other Emacs versions. That's why we now set fail-fast to false.

Additionally, allow_failures from Travis CI is called continue-on-error in Github Actions.

Situation before:

• failure against any emacs version cancels all unfinished jobs
• failure against emacs-snapshot marks the entire test as failed, resulting in:
  • an automatic email sent to me
  • a red "x" mark in the "Actions" tab
  • a red "x" mark in the PR page and commit page

Situation after:

• jobs for all emacs versions run until success/failure, even if another job fails
• failure against emacs-snapshot mostly marks the entire test as passed:
  • no automatic emails (yay)
  • a green "v" mark in the "Actions" tab (yay)
  • a red "x" mark in the PR page and commit page (should be a green "v" or some other suitable indication, waiting on Github to fix this bug, see https://github.com/actions/runner/issues/2347)

This PR seems to fix the CI and the pass/fail marks as much as possible on our side. The remaining misleading red "x" mark is a bug in Github's interface (see https://github.com/actions/runner/issues/2347) .