Only create local data directories if necessary.

Creating a local data directory with Pooch checks that the local
directory is writeable. This is important if downloading data, but if
the files already exist is unnecessary.

This came up because we would like to store data for Pooch in readable
but non-writable directories. In particular, the directories in which
our build system installs Python are not 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,