Set UDUNITS2_XML_PATH in modules (#1843)

This is the "simple" solution to #1842. The ideal way will be to do so via spack-packages, but since spack-stack 2.0 is out soon, this is the "easy" way.

We can undo this once/if spack/spack-packages#2729 is merged and then get into 2.1

Author: mathomp4

configs/common/modules_lmod.yaml

@@ -266,6 +266,12 @@ modules:
           set:
             'UPP_INC': '{prefix}/include'
             'UPP_LIB': '{prefix}/lib/libupp.a'
+      # NOTE: This will be fixed upstream in udunits Spack package
+      # Once this is done, remove this section
+      udunits:
+        environment:
+          set:
+            'UDUNITS2_XML_PATH': '{prefix}/share/udunits/udunits2.xml'
 
       hierarchy:
       - mpi

configs/common/modules_tcl.yaml

@@ -285,3 +285,9 @@ modules:
           set:
             'UPP_INC': '{prefix}/include'
             'UPP_LIB': '{prefix}/lib/libupp.a'
+      # NOTE: This will be fixed upstream in udunits Spack package
+      # Once this is done, remove this section
+      udunits:
+        environment:
+          set:
+            'UDUNITS2_XML_PATH': '{prefix}/share/udunits/udunits2.xml'