(rtl8723au is not an Ethernet driver, but a wireless network adapter driver.)
While you could do complicated things just to get a network connection during the installation process, I suggest to drop that idea and just proceed without network connection. Then, after the installation has finished, boot your fresh installation, install the driver and update packages. This has the same result.
If you would like to have a network connectiong during the installation that requires that driver, you could open a terminal and invoke the commands. Note that a kernel module requires more than just make, you will also need kernel headers matching the target kernel (linux-headers-3....-generic) and a compiler (gcc). The compiler and make are installed with the build-essential package. For offline package installation, see How can I install software or packages without Internet (offline)?
When booting the Live CD, chose for Try instead of Install. You will then start a graphical desktop where you can look for the Terminal application to invoke the commands. If you have a USB, you could then also use the graphical file browser to find out.
Be also aware that manually installing a driver module like that will break every time you update your kernel. To avoid that trouble, you could DKMS to rebuild your module at every update, and/ or file a feature request at the Ubuntu Bugtracker (Launchpad) for driver inclusion (if it was not already available).
The linux-image-extra-3.13.0-30-generic package ships with the rtl8723ae module which should support built-in PCIe cards, so maybe you do not have to go through that trouble of manually installing a driver. Note the e suffix for PCIe, you mentioned the rtl8723au module which exists for USB devices, but maybe that was a typo?
