Add option to limit verbosity of build output

[pablochacin]

When running xk6 in a CI for generating multiple binaries (e.g. for multiple target GOOS and GOARCH) the output is noisy and makes hard to detect errors.

To improve this situation some current log outputs can be moved from INFO to DEBUG. Also a new environment variable XK6_LOG_LEVEL can be added to control amount of output generated.