Thanks for this, this was bugging me for ages (on Mac M1, julia 1.8). I found I also had to set the library path for it to not throw the error about libmpi. For me, I had MPI installed via homebrew, and the following steps worked (in case anyone else had similar issues):
export JULIA_MPI_PATH=/opt/homebrew/Cellar/open-mpi/4.1.4
export JULIA_MPI_LIBRARY=/opt/homebrew/Cellar/open-mpi/4.1.4/lib/libmpi
julia -e 'ENV["JULIA_MPI_BINARY"]="system"; using Pkg; Pkg.build("MPI"; verbose=true)'
This resulted in the MPI.toml file (you can find in .julia/prefs) being (automatically) set up as:
library = "/opt/homebrew/Cellar/open-mpi/4.1.4/lib/libmpi"
binary = "system"
path = "/opt/homebrew/Cellar/open-mpi/4.1.4"
and I was able to run mpiexec/mpirun as expected with julia.
Also note, if I did the above, but with e.g. “/opt/homebrew/lib” it did not work, even though there are libmpi.dylib files there. The issue, I believe, is these are aliases, and if you do ‘show original’ it takes you to the path I used above.