Problem: Flux expects the `FLUX_PMI_LIBRARY_PATH` to point directly at
the `libpmi.so` installed by Flux. When the env var is unset,
prepending to it results in this behavior. In the rare case that the
env var is already set, then the spack `libpmi.so` gets prepended with a
`:`, which Flux then attempts to interpret as a single path.
Solution: don't prepend to the path, instead set the path to point to
the `libpmi.so` (which will be undone when Flux is unloaded).
* flux-core: remove deprecated environment variables
The earliest checksummed version in this package is 0.15.0. As of
0.12.0, wreck (and its associated paths) no longer exist in Flux. As of
0.13.0, the `FLUX_RCX_PATH` variables are no longer used. So clean up
these env vars from the `setup_run_environment`.
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