use a shorter log format without timestamp on github actions

[fwilhe]

What happened:

The builder prepends log lines with a output file name and a time stamp.
As some CI systems (notably GitHub actions) also log a timestamp by default, the line is cluttered with two time stamps, leaving litte screen space for the actual log message.

See this screenshot as an example:

What you expected to happen:

Either automatically detect when we're running on a known CI system that has timestamps in the log and omit them, or have an option/environment variable to build shorter log prefixes.