FindMPI: make sure computed link flags are not de-duplicated

In commit f7eaa342 (FindMPI: Store imported target link flags as a
list instead of a string, 2019-06-14, v3.15.0-rc2~2^2) we used
`separate_arguments` to parse the extracted link flags and add them to
`INTERFACE_LINK_LIBRARIES`.  That property is not meant for link flags
and CMake may de-duplicate them.  This is particularly problematic for
flags like `-Wl,-rpath -Wl,/path1 -Wl,-rpath -Wl,/path2`.

In commit 39c572c9 (FindMPI: Updated to use INTERFACE_LINK_OPTIONS,
2019-06-24) we moved the parsed flags over to `INTERFACE_LINK_OPTIONS`,
but that may still perform de-duplication.  Avoid the parsing and
de-duplication of flags by passing the original string via `SHELL:`
instead.

Fixes: #19516
7 jobs for correct_find_mpi_incorrect_deduplication
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