zos,test: fix spawn_empty_env for shared library build
zos,test: fix spawn_empty_env for shared library build

z/OS uses LIBPATH instead of LD_LIBRARY_PATH for the search path
for shared libraries.

PR-URL: https://github.com/libuv/libuv/pull/2737
Reviewed-By: Ben Noordhuis <59bd0a3ff43b32849b319e645d4798d8a5d1e889@bnoordhuis.nl>

unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso