pw_build: Add dep for copy_from_cipd

Make sure the copy_from_cipd script is fully installed before using it.

Change-Id: I930ef3ae2c5d05d2e70f7193d2923b0ca757087e
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/41661
Pigweed-Auto-Submit: Rob Mohr <mohrr@google.com>
Reviewed-by: Wyatt Hepler <hepler@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>
diff --git a/pw_build/copy_from_cipd.gni b/pw_build/copy_from_cipd.gni
index bb9502b..19d332a 100644
--- a/pw_build/copy_from_cipd.gni
+++ b/pw_build/copy_from_cipd.gni
@@ -75,6 +75,8 @@
     # well.
     inputs = [ invoker.manifest ]
     outputs = [ _out_file ]
+
+    python_deps = [ "$dir_pw_build/py" ]
   }
 
   pw_source_set(target_name) {