Pass go build arguments from env var

[imiric]

Currently changing go build arguments is very limited, and only -race is exposed via XK6_RACE_DETECTOR.

It would be useful to pass a generic env var like XK6_BUILD_ARGS to go build instead, so that, for example, debug symbols can be optionally included if desired, as we recently discussed in Slack. We should get rid of XK6_RACE_DETECTOR in that case, since it could be passed via XK6_BUILD_ARGS.