function getEnv(key, options) {
    const env = (options && options.env) || process.env
    if (options && options.excludeNpm === true) {
        if (key.startsWith('npm_config_')) {
            return ''
        }
    }
    if (options && options.lowerCaseOnly === true) {
        return env[key.toLowerCase()] || ''
    } else if (options && options.favourUpperCase === true) {
        return env[key.toUpperCase()] ||  env[key.toLowerCase()] || ''
    }
    return env[key.toLowerCase()] || env[key.toUpperCase()] || ''
}
Get the value for an environment constiable.

@param {string} key - The name of the environment constiable.
@param {ProxyOptions} options - The name of the environment constiable.
@return {string} The value of the environment constiable.
@private