libplacebo-ubuntu-cosmic: fix installation of shaderc .so

libshaderc_shared.so is a symlink to libshaderc_shared.so.1. install aggressively dereferences symlinks, so it ended up installing the .so.1 to the path of .so, which worked while resolving dependencies but broke at link time - resulting in a failed job.

Fix this issue by just using cp -a instead - the permissions are correct by default anyway.