scripts: update open calls to use explicit encoding

BUG=b:187789896
TEST=`cros lint` is clean here

Change-Id: Ia54475869c17292505537334362cd4cff6438b14
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/4290041
Commit-Queue: Cindy Lin <xcl@google.com>
Reviewed-by: Cindy Lin <xcl@google.com>
Tested-by: Mike Frysinger <vapier@chromium.org>
Auto-Submit: Mike Frysinger <vapier@chromium.org>
diff --git a/scripts/upload_prebuilts.py b/scripts/upload_prebuilts.py
index aa561f3..f90bcff 100644
--- a/scripts/upload_prebuilts.py
+++ b/scripts/upload_prebuilts.py
@@ -1158,5 +1158,5 @@
         )
 
     if options.output:
-        with open(options.output, "w") as f:
+        with open(options.output, "w", encoding="utf-8") as f:
             pformat.json(report, fp=f)