Cygwin shell doesn't support inline environment variables

i.e. `MYSQL_PWD=123 mysql` causes a "MYSQL_PWD is not recognized as an internal or external command." error

More info: http://wordpress.stackexchange.com/questions/136348/wp-cli-0-14-1-mysql-error

So, it looks like it's back to `putenv()`: https://github.com/wp-cli/wp-cli/issues/950#issuecomment-35818536