4.4: Improve CI and setup with Bash docker image

[jansorg]

• This brings back stable CI by working around bash 4.4: jobs is unavailable tianon/docker-bash#44
• It also uses env bash for check-prefix.sh to make it work better in Bash docker images.
• Enables manual execution of the GitHub Actions workflow

Same as #59, but for bash-4.4

[jansorg]

@rocky Merging this now, as it should be safe and I'd like to use the bash-4.4 branch for my forked repository.
Happy to revert if you'd like to review.

[rocky]

This is fine. mean to look at it over the weekend.