ci(github): also mark up compile errors

When GCC produces those helpful errors, we will want to present them in the GitHub workflow runs in the most helpful manner. To that end, we want to use workflow commands to render errors and warnings: https://docs.github.com/en/actions/using-workflows/workflow-commands-for-github-actions In the previous commit, we ensured that grouping is used for the build in all jobs, and this allows us to piggy-back onto the `group` function to transmogrify the output. Note: If `set -o pipefail` was available, we could do this in a little more elegant way. But since some of the steps are run using `dash`, we have to do a little `{ ...; echo $? >exit.status; } | ...` dance. Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>