Allow compiling with debug info

[pablochacin]

xk6 builder has hardcoded the options for removing debug info:

"-ldflags", "-w -s", // trim debug symbols

This prevent using debugging tools. For example, dlv throws the following error message:

could not launch process: decoding dwarf section info at offset 0x0: too short - debuggee must not be built with 'go run' or -ldflags='-s -w', which strip debug info

It would be convenient to add a flag or environment variable for changing this behavior.