Fix nimbus URL joining (#1997)

### What kind of change does this PR introduce?

* Fixed an issue that was causing URLs to include a `//` between the
repository and branch fields.

### Does this PR introduce a breaking change?

No.

### Other information:

While this was working fine for returning files already available on
disk, when `nimbus` was fetching files remotely when launched from a
Jupyter notebook, this was failing.

Related: Ouranosinc/xscen#492

CHANGELOG.rst

@@ -13,6 +13,7 @@ Breaking changes
 Bug fixes
 ^^^^^^^^^
 * Fixed pickling issue with ``xclim.sdba.Grouper`` and other classes for usage with `dask>=2024.11`. (:issue:`1992`, :pull:`1993`).
+* Fixed an issue with ``nimbus`` that was causing URL path components to be improperly joined. (:pull:`1997`).
 
 Internal changes
 ^^^^^^^^^^^^^^^^

xclim/testing/utils.py

@@ -435,7 +435,12 @@ def load_registry(
     dict
         Dictionary of filenames and hashes.
     """
-    remote_registry = audit_url(f"{repo}/{branch}/data/registry.txt")
+    remote_registry = audit_url(
+        urljoin(
+            urljoin(repo, branch if branch.endswith("/") else f"{branch}/"),
+            "data/registry.txt",
+        )
+    )
 
     if branch != default_testdata_version:
         custom_registry_folder = Path(
@@ -511,8 +516,9 @@ def nimbus(  # noqa: PR01
             "The `pooch` package is required to fetch the xclim testing data. "
             "You can install it with `pip install pooch` or `pip install xclim[dev]`."
         )
-
-    remote = audit_url(f"{repo}/{branch}/data")
+    remote = audit_url(
+        urljoin(urljoin(repo, branch if branch.endswith("/") else f"{branch}/"), "data")
+    )
     return pooch.create(
         path=cache_dir,
         base_url=remote,