ARROW-12320: [CI] REPO arg missing from conda-cpp-valgrind

Closes #9973 from westonpace/bugfix/arrow-12320

Authored-by: Weston Pace <weston.pace@gmail.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>
diff --git a/docker-compose.yml b/docker-compose.yml
index 683ab88..539d5ad 100644
--- a/docker-compose.yml
+++ b/docker-compose.yml
@@ -230,6 +230,7 @@
       cache_from:
         - ${REPO}:${ARCH}-conda-cpp
       args:
+        repo: ${REPO}
         arch: ${ARCH}
         prefix: /opt/conda
     shm_size: *shm-size