When shared mode is active, set cache location to SPACK_PATH

lib/spack/spack/caches.py

@@ -40,7 +40,12 @@ def _fetch_cache():
     This prevents Spack from repeatedly fetch the same files when
     building the same package different ways or multiple times.
     """
+    if spack.config.get('config:shared'):
+        path = os.path.join(os.environ['SPACK_PATH'],
+                            'var/spack/cache')
+    else:
         path = spack.config.get('config:source_cache')
+
     if not path:
         path = os.path.join(spack.paths.var_path, "cache")
     path = canonicalize_path(path)