Add module purge to beginning of orion build module, update regiona…

…l_workflow hash (#206)

## DESCRIPTION OF CHANGES: 
With changes in regional_workflow necessary for fixing of some problems on the Cheyenne platform, the ufs-srweather-app hash needs to be updated, and a `module purge` command needs to be added to the beginning of the Orion platform's build module. This ensures a clean and consistent build environment and avoids tricky-to-solve environment-specific build errors.

## TESTS CONDUCTED: 
Testing complete on Cheyenne, Orion, Hera, and Jet. All tests pass aside from those with pre-existing issues; see ufs-community/regional_workflow#672

## ISSUE: 
Related to issue ufs-community/regional_workflow#663

Externals.cfg

@@ -3,7 +3,7 @@ protocol = git
 repo_url = https://github.com/ufs-community/regional_workflow
 # Specify either a branch name or a hash but not both.
 #branch = develop
-hash = 20a149d
+hash = f5f8158
 local_path = regional_workflow
 required = True
 

env/build_orion_intel.env

@@ -1,5 +1,7 @@
 #Setup instructions for MSU Orion using Intel-18.0.5 (bash shell)
 
+module purge
+
 module load contrib noaatools
 
 module load cmake/3.18.1