mrustc's README says "make -f minicargo.mk - Builds mrustc and minicargo, then builds libstd, libtest, finally rustc and cargo". I don't think this project actually uses the rustc and cargo that get built, it just uses mrustc + minicargo. If that's the case, then I think you could change build.sh to run make -f minicargo.mk LIBS (per the mrustc README) and skip building rustc and cargo binaries.
mrustc's README says "make -f minicargo.mk - Builds mrustc and minicargo, then builds libstd, libtest, finally rustc and cargo". I don't think this project actually uses the rustc and cargo that get built, it just uses mrustc + minicargo. If that's the case, then I think you could change build.sh to run
make -f minicargo.mk LIBS
(per the mrustc README) and skip building rustc and cargo binaries.