[contrib] Displaying the time spent in the guix build #24011

Add the function of displaying the time spent in the guix build.

Guix building process takes a long time in the normal desktop pc, and this value can help in the work about improving performance.

Example, after complete building process of guix-build: ... Elapsed time: 120007 milliseconds (~2 minute)

Thanks, but this change is over-complicated for what is trying to be achieved. Anyone who wants to time their build can just run time ./contrib/guix/guix-build. Builders can decide if they want the timing output, and that way it isn't restricted to the guix-build script.