Fix gtest cp -p flag

As reported by @manu:

why you enforce -p flag for 'cp' ?
https://gitlab.isc.org/isc-projects/kea/-/blob/master/ext/gtest/Makefile.am#L10
it would be good idea to remove it as it causes errors

cp -p /usr/src/googletest/googletest//src/gtest-all.cc gtest-all.cc
cp: can't preserve ownership of 'gtest-all.cc': Operation not permitted
as we copy file owned by root to jenkins's workspace under jenkins user. The file is copied anyway but without preserved ownership which we don't need, right? So the -p flag is not required from my point of view.