Allow setting environment variable in csharp_test

[titusfortner]

I'm looking to create a set of targets that toggle on a feature. In my test code I have it set to toggle based on the presence of an Environment variable. If you can add an env attribute to pass it in, that would be very helpful.

I'm not certain the right syntax, but based on what I have working in rules_ruby, I think it would look like:

                csharp_test(
                    name = bidi_test_name,
                    srcs = lib_srcs + [src] + ["@rules_dotnet//dotnet/private/rules/common/nunit:shim.cs"],
                    deps = deps + extra_deps,
                    env =  {"WEBDRIVER_BIDI": "true"},
                    target_frameworks = target_frameworks,
                    args = _BROWSERS[browser]["args"] + _HEADLESS_ARGS,
                    data = data + _BROWSERS[browser]["data"],
                    tags = tags + [browser] + COMMON_TAGS + _BROWSERS[browser]["tags"],
                    size = size,
                    **kwargs
                )
                tests.append(bidi_test_name)

[purkhusid]

This is a reasonable request, I'll see if I can add it when I have time.