Only create local data directories if necessary (#370)

Creating a local data directory with Pooch checks that the local
directory is writable. This is important if downloading data, but if
the files already exist is unnecessary. Doing so makes it impossible
to store data for Pooch in readable but non-writable directories. 
In particular, the directories in which some build systems install
Python may not be writable.

pooch/core.py

@@ -217,15 +217,17 @@ def retrieve(
         path = os_cache("pooch")
     if fname is None:
         fname = unique_file_name(url)
-    # Create the local data directory if it doesn't already exist and make the
-    # path absolute.
+    # Make the path absolute.
     path = cache_location(path, env=None, version=None)
-    make_local_storage(path)
 
     full_path = path.resolve() / fname
     action, verb = download_action(full_path, known_hash)
 
     if action in ("download", "update"):
+        # We need to write data, so create the local data directory if it
+        # doesn't already exist.
+        make_local_storage(path)
+
         get_logger().info(
             "%s data from '%s' to file '%s'.",
             verb,
@@ -560,9 +562,6 @@ def fetch(self, fname, processor=None, downloader=None, progressbar=False):
         """
         self._assert_file_in_registry(fname)
 
-        # Create the local data directory if it doesn't already exist
-        make_local_storage(str(self.abspath))
-
         url = self.get_url(fname)
         full_path = self.abspath / fname
         known_hash = self.registry[fname]
@@ -574,6 +573,10 @@ def fetch(self, fname, processor=None, downloader=None, progressbar=False):
             )
 
         if action in ("download", "update"):
+            # We need to write data, so create the local data directory if it
+            # doesn't already exist.
+            make_local_storage(str(self.abspath))
+
             get_logger().info(
                 "%s file '%s' from '%s' to '%s'.",
                 verb,