Fix environment variable name in global_cycle.orion.lua

Fixes #123

Author: GeorgeGayno-NOAA

modulefiles/fv3gfs/global_cycle.orion.lua

@@ -10,7 +10,7 @@ load(pathJoin("hpc", hpc_ver))
 hpc_intel_ver=os.getenv("hpc_intel_ver") or "2018.4"
 load(pathJoin("hpc-intel", hpc_intel_ver))
 
-hpc_intel_ver=os.getenv("hpc_impi_ver") or "2018.4"
+hpc_impi_ver=os.getenv("hpc_impi_ver") or "2018.4"
 load(pathJoin("hpc-impi", hpc_impi_ver))
 
 w3nco_ver=os.getenv("w3nco_ver") or "2.4.1"