nix build --json: Include build statistics

[edolstra]

Extracted from #3600 (which returns some statistics when cgroups are enabled). Currently, it only returns startTime and stopTime.

Example:

# nix build -L --extra-experimental-features cgroups --impure --expr 'with import <nixpkgs> {}; runCommand "foo" {} "dd if=/dev/urandom bs=1M count=1024 | md5sum; mkdir $out"' --json
[
  {
    "cpuSystem": 1.911431,
    "cpuUser": 1.214249,
    "drvPath": "/nix/store/xzdqz67xba18hljhycp0hwfigzrs2z69-foo.drv",
     "outputs": {
      "out": "/nix/store/rh9mc9l2gkpq8kn2sgzndr6ll7ffjh6l-foo"
    },
    "startTime": 1669024076,
    "stopTime": 1669024079
  }
]