INSTALL: `mpirun` not symlinked by `firedrake-install`

[JDBetteridge]

Describe the error
mpiexec is symlinked to the MPI that the Firedrake install script installs (or is pointed to), but not mpirun and possibly other binaries in the mpi*/bin directory.

Steps to Reproduce
Steps to reproduce the behaviour:

1. Install Firedrake and let PETSc build MPICH. (firedrake-install)
2. mpirun -n 2 python -c "from mpi4py import MPI; print(MPI.COMM_WORLD.rank, MPI.COMM_WORLD.size)"

Expected behavior

0 2
1 2

Error message

0 1
0 1

Environment:

• N/A

Additional context
@jshipton @tommbendall (re?)-discovered this bug

[adofarsi]

Is #3028 related to this issue?

[JDBetteridge]

No, I will respond on 3028 though