Some environment variable (e.g., `LD_PRELOAD`) set via `run_options` don't have the desired effect.

[stevenjswanson]

This code:

with local_execution():
    alloc_run = run(alloc, "go", arg_map(size=4, count=100000000), run_options=arg_map(LD_PRELOAD=["libtcmalloc.so",""])

does not work be we currently set the environment variables inside the executor rather outside the executor.

This makes it awkward to run an experiment comparing different implementations of the malloc()/free().

[stevenjswanson]

Looked into this some. It's easy enough to set environment before we invoke cfiddle-run to run the command, but the problem is those different invocations what different values, so we'd have to invoke cfiddle-run for each run. This might be inefficient, and definitely requires some more retooling of how we do execution.