Do not require mpi to be runnable

[dschwoerer]

This fails on many login nodes of HPC systems.
Typically running mpi on the login nodes is disabled, or doesn't work as the ISA is different compared to the compute nodes.

Currently I get an error like this:

-- Performing Test MULTIPASS_TEST_4_petsc_works_all
-- Performing Test MULTIPASS_TEST_4_petsc_works_all - Failed
-- PETSc could not be used, maybe the install is broken.
CMake Error at /u/dave/.local/lib/python3.6/site-packages/cmake/data/share/cmake-3.21/Modules/FindPackageHandleStandardArgs.cmake:230 (message):
  PETSc could not be found.  Be sure to set PETSC_DIR and PETSC_ARCH.
  (missing: PETSC_EXECUTABLE_RUNS) (found version "3.16.4")
Call Stack (most recent call first):
  /u/dave/.local/lib/python3.6/site-packages/cmake/data/share/cmake-3.21/Modules/FindPackageHandleStandardArgs.cmake:594 (_FPHSA_FAILURE_MESSAGE)
  cmake/FindPETSc.cmake:383 (find_package_handle_standard_args)
  cmake/SetupBOUTThirdParty.cmake:95 (find_package)
  CMakeLists.txt:413 (include)