biology/linux-foldingathome: Fix fetch on amd64

I forgot to group MASTER_SITES into the right groups. During the fix,
however, I realized that we can simplify the whole mechanism by keeping
not only the base name but also some part of the directory path in
DISTNAME. This way names of DISTFILES are unique across architectures.

Reported by: pkg-fallout
Fixes: 6086ff0b3283 biology/linux-foldingathome: Add support for aarch64