[script] exclude ot_testing and __pycache__ when building the OTBR docker (#9457)

This commit excludes `ot_testing` and `__pycache__` directories when
copying the files before building the docker.

Such files are usually generated by root user and will cause errors
when copying them as a non-root user.

script/test

@@ -382,6 +382,8 @@ do_build_otbr_docker()
         rm -rf third_party/openthread/repo
         rsync -r \
             --exclude=build \
+            --exclude=ot_testing \
+            --exclude=__pycache__ \
             "${otdir}/." \
             third_party/openthread/repo
         rm -rf .git