Prepend 'exec 2>&1' to custom_script to untangle output

[erikmd]

• This patch workarounds GitHub Actions unflushed std(out/err) output.

[Zimmi48]

It's hard for me to review because I didn't know about the issue and I don't understand the fix. But I trust you on this.

[erikmd]

OK, so FTR: the fact is that the lines automatically added by set -x are sent to stderr, while the main output is sent to stdout, and the issue was that GHA does not seem to flush printing to these two different channels, resulting (before this PR) is a distracting un-sync.

For example, beforehand:
https://github.com/erikmd/docker-coq-github-action-demo/runs/1456333460?check_suite_focus=true#step:4:188

some value
+ (script @ line 4) $ printf '%s\n' 'some value'

henceforth:
https://github.com/erikmd/docker-coq-github-action-demo/runs/1456454613?check_suite_focus=true#step:4:188

+ (script @ line 4) $ printf '%s\n' 'some value'
some value

[Zimmi48]

OK, so basically, you redirect them to the same channel to ensure they are printed in order. Sounds good to me. However, this might also be worth to open a feedback ticket with GitHub about this: https://support.github.com/contact/feedback

[erikmd]

this might also be worth to open a feedback ticket with GitHub about this: https://support.github.com/contact/feedback

Done.