Track engine compilation times and peak memory usage during build

[Calinou]

If the benchmark server builds Godot from source, we should find a way to track engine compilation times to check for regressions on that aspect.

It's also worth measuring the peak memory usage of SCons and all its child processes (not the entire system), as we should aim to keep Godot buildable on systems with only 8 GB of RAM for the foreseeable future (at least without LTO).

[Calinou]

Build times are now tracked by run-benchmarks.sh in https://github.com/Calinou/godot-benchmarks/tree/add-web-interface. I haven't looked at tracking peak compiler memory usage yet.