wreck/openmpi.lua: drop LD_LIBRARY_PATH, add TMPDIR

The new openmpi Flux component uses FLUX_PMI_LIBRARY_PATH
at runtime to dlopen our PMI library, so LD_LIBRARY_PATH
does not need to be set for it to work right in the
default configuration.  Therefore, don't change it
in the openmpi lua script.

Do set TMPDIR to be the rank-specific directory so that
shared memory segment names do not collide when
there are multiple Flux ranks on a (real) node launching
an MPI program with multiple MPI ranks on a (flux) node.

src/modules/wreck/lua.d/openmpi.lua

@@ -19,13 +19,9 @@ local dirname = require 'flux.posix'.dirname
 function rexecd_init ()
     local env = wreck.environ
     local f = wreck.flux
-    local libpmi = f:getattr ('conf.pmi_library_path')
-    local ldpath = dirname (libpmi)
+    local rankdir = f:getattr ('scratch-directory-rank')
 
-    if (env['LD_LIBRARY_PATH'] ~= nil) then
-        ldpath = ldpath..':'..env['LD_LIBRARY_PATH']
-    end
-    env['LD_LIBRARY_PATH'] = ldpath
+    env['TMPDIR'] = rankdir
 end
 
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