Fix path to build directory for downstream projects

Fixes #5749.
diff --git a/buildkite/bazelci.py b/buildkite/bazelci.py
index b4d3bef..790d853 100644
--- a/buildkite/bazelci.py
+++ b/buildkite/bazelci.py
@@ -292,7 +292,7 @@
         "windows": {
             "name": "Windows",
             "emoji-name": ":windows:",
-            "agent-directory": "c:/build/${BUILDKITE_AGENT_NAME}",
+            "agent-directory": "d:/build/${BUILDKITE_AGENT_NAME}",
         }
     }