Allow setting updated/new env var *values* on Runner.

In _utils.py the create_environ() helper produces a dict that can be passed as the "env" arg to subprocess.run(), etc. It is used in Master.spawn_worker() (in _master.py). Currently it takes an "inherit_environ" arg (Bool) that corresponds to the "--inherit-environ" flag in the Runner CLI (see Runner.init() in _runner.py). This results in a potentially problematic situation.

The Problem

Let's say you have a benchmark script that internally relies on some environment variable that is defined relative to the commandline args given to the script. This environment variable may be set already or it might not. Regardless, you will be setting it to some new value. To make this work you need to do something like the following:

    # This is a concrete example.
    os.environ["MY_VAR"] = "spam" if runner.args.my_flag else "eggs"
    if runner.args.inherit_environ is None:
        runner.args.inherit_environ = ["MY_VAR"]
    else:
        runner.args.inherit_environ.append("MY_VAR")

However, in some cases you can't leave the env var set (or maybe the env var could cause pyperf to break). Plus things are more complicated if you have more than one such env var.

The Solution

Consequently, in a benchmark script it would be nice to be able to give the actual env var pairs to Runner rather than doing the dance above. Here are some possible approaches to solve the problem:

• allow Runner.args.inherit_environ to be a dict
  • create_environ() would be updated to do the right thing
  • this is probably too messy to be part of pyperf's public API
• add something like Runner.env_vars to allow benchmarks to explicitly set env var values to be used in workers
  • create_environ() would grow a new "env" arg or "inherit_environ" would updated as above
• add Runner.add_env_var(name, value=INHERIT)
  • this is like either of the two above, but probably a better public API for the functionality

Start by reading create_environ() in _utils.py, its use in Master.spawn_worker() in _master.py, and Runner.init() in _runner.py. Decide how Runner should accept explicit environment values, then ensure workers receive those values without requiring benchmark code to mutate os.environ; the existing inherit-environ behavior should remain accounted for.