[llvm-exegesis] Unbreak `--benchmarks-file=<f>`

I'm absolutely flabbergasted by this.
I was absolutely sure this worked.
But apparently not.

When outputting to the file, we'd write a single `InstructionBenchmark`
to it, and then close the file. And for the next `InstructionBenchmark`,
we'd reopen the file, truncating it in process,
and thus the first `InstructionBenchmark` would be gone...