Fix reading .ci.yaml in windows.

The checkout of repos with names like mirrors/flutter is translated as
mirrors\flutter in windows and it was causing issues to find the file
for reading.

Bug: https://github.com/flutter/flutter/issues/116795
Change-Id: I0b5069d18a5d414aac4d74437a16f3be369c3771
Reviewed-on: https://flutter-review.googlesource.com/c/recipes/+/37120
Reviewed-by: Drew Roen <drewroen@google.com>
Commit-Queue: Godofredo Contreras <godofredoc@google.com>
diff --git a/recipes/release/release_builder.py b/recipes/release/release_builder.py
index 29ed40e..a96822f 100644
--- a/recipes/release/release_builder.py
+++ b/recipes/release/release_builder.py
@@ -58,7 +58,8 @@
 
 def RunSteps(api, properties, env_properties):
   repository = api.buildbucket.gitiles_commit.project
-  checkout_path = api.path['start_dir'].join(repository)
+  repository_parts = repository.split('/')
+  checkout_path = api.path['start_dir'].join(*repository_parts)
   git_ref = api.properties.get('git_ref') or api.buildbucket.gitiles_commit.ref
   git_url = api.properties.get('git_url') or REPOS[repository]
   api.repo_util.checkout(