You could try running a Raspberry but add a Network Block device from another machine and. (xNBD: https://bitbucket.org/hirofuchi/xnbd/wiki/Home )

Then export a RAM disk from that machine and add the NBD disk as swap on the Raspberry. It would be slow, but builds would complete. Then you'd need only one low-to-moderate power machine (a PC presumably) in your Raspberry cluster, just with lots of RAM in the one PC.