get env() {
        // Load .env, if it exists, into process.env
        expand(dotenv.config({ path: this.envPath }));

        // Take everything from process.env that beings with MIX_
        const regex = /^MIX_/i;
        const existing = Object.fromEntries(
            Object.entries(process.env).filter(([key]) => regex.test(key))
        );

        // Merge in env vaiues from:
        // - process.env
        // - the .env file
        // - the additional env provided to the plugin
        return {
            ...existing,
            ...this.additionalEnv
        };
    }
Build all MIX_ definitions for Webpack's DefinePlugin.