Re: [PATCH 2/2] ci(github): also mark up compile errors

"Johannes Schindelin via GitGitGadget" [off-list ref]
writes:

+		sed 's/^\(\([^ ]*\):\([0-9]*\):\([0-9]*:\) \)\(error\|warning\): /::\5 file=\2 line=\3::\1/'

https://docs.github.com/en/actions/using-workflows/workflow-commands-for-github-actions#setting-a-warning-message

seems to use comma as field separator, not SP, for files and lines.
We'll see if they are equivalent without getting documented soon, as
I will be adding this to my tree for 'seen' today.  We should throw
a build failure in to see its effect ;-)