Fix exported PATH variables to prepend custom OMPI

to make sure to not use the default system MPI installation.

rank-swapper-agent/Makefile

 export OMPI := /home/openmpi-install
-export PATH := $(PATH):$(OMPI)/bin
-export LD_LIBRARY_PATH := $(LD_LIBRARY_PATH):$(OMPI)/bin
+# BEWARE: the order (custom OpenMPI first/prepended) is important since the
+# first gets picked and there may be other MPI installations on the system!
+export PATH := $(OMPI)/bin:$(PATH)
+export LD_LIBRARY_PATH := $(OMPI)/bin:$(LD_LIBRARY_PATH)
 # requirement: GNU make, see https://unix.stackexchange.com/questions/11530/adding-directory-to-path-through-makefile/261844#261844
 
 CC = gcc

rank-swapper-agent/client.sh

 #!/bin/bash
 
-export OMPI=/home/openmpi-install
-export DPM_AGENT_SOCKET=$OMPI/dpm_socket_socket
-export PATH=$PATH:$OMPI/bin
-export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:$OMPI/bin
+# the export not done directly since not all shells allow the value of a
+# variable to be set at the same time it is exported
+OMPI=/home/openmpi-install; export OMPI
+DPM_AGENT_SOCKET=$OMPI/dpm_socket_socket; export DPM_AGENT_SOCKET
+# BEWARE: the order (custom OpenMPI first/prepended) is important since the
+# first gets picked and there may be other MPI installations on the system!
+PATH=$OMPI/bin:$PATH; export PATH
+LD_LIBRARY_PATH=$LD_LIBRARY_PATH:$OMPI/bin; export LD_LIBRARY_PATH
 
 PROCESSES=1