`maxrss` is skewed if `bench.zig` needs to be compiled

[Ryan Liptak]

I've been trying to make gotta-go-fast more consistent, and have been testing by running the benchmarks for a single commit multiple times. I noticed that on the first run, if bench.zig needs to be compiled, maxrss will be much larger than it will on subsequent runs:

benchmark	maxrss run 1	maxrss run 2	maxrss run 3
self-hosted-parser	534872	95520	95284
zig-fmt	406304	97664	97744
translate-c-windows-h	408728	69344	69692
arena-allocator	461844	95512	94976
std-rand	461384	97816	97820
std-hash-map	460512	96332	95488
insert-10M-int	452404	221248	221248
random-distinct	453284	110736	110728
random-find	460872	99680	95232
ast-check-os	404396	69016	68724
ast-check-AstGen	404984	68940	69092
ast-check-Sema	405624	69432	69164

One extremely simple but quite wasteful fix would be to add

{
    const warmup_run = try execCapture(gpa, main_argv.items, .{ .cwd = bench_cwd });
    defer gpa.free(warmup_run);
}

before the const main_stdout = try execCapture(...) call.