Drop recipe default environment variables.
Drop recipe default environment variables.

All of these are set effectively through the `toolchain`. So there is no
further need to hard code them in the build script.

nanshe-org/rank_filter,jakirkham/rank_filter,nanshe-org/rank_filter,DudLab/rank_filter,DudLab/rank_filter,DudLab/rank_filter,jakirkham/rank_filter,jakirkham/rank_filter,nanshe-org/rank_filter