There is no "solution" to speak of, tmpfs can be mounted on any directory on linux, so there is no difference to normal build process at least on linux. Also for example I can serialise build jobs in jenkins, so the total amount of space required will be spread over time.
RAM is more expensive than SSDs but it's still very cheap at roughly $5 per GB. Higher density 32GB DIMMs cost only a bit more at $7 per GB. You can buy terabytes of RAM for a few thousand dollars.
It may be far cheaper to do so than to spend developer time coming up with a ramdisk solution. And RAM is far more expensive, per GB, than SSDs.
Next year's drives will be cheaper and better, anyway.