Log Message:
math/four-bar-rs: fix build

Pin original release commit to fix our builds.
v7.0.1 was originally released to fix NetBSD builds see:
https://github.com/KmolYuan/four-bar-rs/issues/5

Unfortunately, upstream decided to push several other commits onto that tarball.
This causes build failures at our end ... again see:
https://github.com/KmolYuan/four-bar-rs/issues/6