CI: upgrade actions/cache from v2 to v4

See actions/toolkit#1890

Author: ctrueden

.github/workflows/build.yml

@@ -71,7 +71,7 @@ jobs:
     steps:
       - uses: actions/checkout@v2
       - name: Cache conda
-        uses: actions/cache@v2
+        uses: actions/cache@v4
         env:
           # Increase this value to reset cache if dev-environment.yml has not changed
           CACHE_NUMBER: 0