Update cache from v2 to v4

Author: forsyth2

.github/workflows/build_workflow.yml

@@ -24,7 +24,7 @@ jobs:
           fetch-depth: 0
 
       - name: Cache Conda
-        uses: actions/cache@v2
+        uses: actions/cache@v4
         env:
           # Increase this value to reset cache if conda-env/docs.yml has not changed in the workflow
           CACHE_NUMBER: 0