Allow undefined env var values

[AArnott]

While it doesn't make any sense, the typings for process.env is:

    interface ProcessEnv {
        [key: string]: string | undefined;
    }

So assigning process.env to IExecSyncOptions.env in typescript with all typings turned on produces compiler errors (at least when strict null checking is applied).

Without this simple typings adjustment, the following workaround in user code is required:

function GetFilteredEnv() {
    const result = {};
    const block = process.env;
    for (const key in block) {
        if (block.hasOwnProperty(key)) {
            const value = block[key];
            if (value !== undefined) {
                result[key] = value;
            }
        }
    }
    return result;
}

let options: IExecSyncOptions = { env: GetFilteredEnv() };

But with this change it's as simple as:

let options: IExecSyncOptions = { env: process.env };

Which ironically is already what is done within this repo, but isn't a problem because the node typings aren't imported so process.env is typed as any.

[AArnott]

The failure on Linux appears to be an unrelated (network?) failure that I see popping up in several other builds as well including a recent CI on master.

[damccorm]

LGTM - thanks for the contribution!