Fall back to looking for the MPI command in the putative Conda binary directory

If `$CONDA_PREFIX/bin` is not in the `PATH`, then the putative Conda binary
directory is the same as the directory the Khiops `MODL[_openmpi|mpich]` binary
is situated in.

Note: `KHIOPS_MPI_COMMAND` might still contain the incorrect `mpiexec`,
in the following (rather unlikely) scenario:
- `khiops_env` is run in a "Conda-based" environment
- there is a preexisting system-wide MPI installation which contains
  `mpiexec`.
This should be mitigated following the Khiops Python issue
KhiopsML/khiops-python#301.

closes #476

Author: popescu-v

packaging/linux/common/khiops_env/khiops_env.in

@@ -84,8 +84,17 @@ if command -v mpiexec &>/dev/null; then
     KHIOPS_MPI_ERROR=""
     KHIOPS_MPI_COMMAND="$(type -P mpiexec) $MPI_EXTRA_FLAGS @KHIOPS_MPI_EXTRA_FLAG@ @MPIEXEC_NUMPROC_FLAG@ $KHIOPS_PROC_NUMBER"
 else
-    KHIOPS_MPI_ERROR="We didn't find mpiexec in the regular path. Parallel computation is unavailable: Khiops is launched in serial"
-    KHIOPS_MPI_COMMAND=""
+    # Fallback for Conda-based environments where `mpiexec` is not in PATH,
+    # because $CONDA_PREFIX/bin is not in PATH
+    _MPIEXEC=@KHIOPS_PATH@mpiexec
+    if command -v $_MPIEXEC &>/dev/null; then
+        KHIOPS_MPI_ERROR=""
+        KHIOPS_MPI_COMMAND="$_MPIEXEC $MPI_EXTRA_FLAGS @KHIOPS_MPI_EXTRA_FLAG@ @MPIEXEC_NUMPROC_FLAG@ $KHIOPS_PROC_NUMBER"
+    else
+        KHIOPS_MPI_ERROR="We didn't find mpiexec in the expected paths. Parallel computation is unavailable: Khiops is launched in serial"
+        KHIOPS_MPI_COMMAND=""
+    fi
+    unset _MPIEXEC
 fi
 
 # without more than 2 procs, we use the serial khiops