Aha, never mind... I am using the Rust "coreutils" and this appears to be an incompatibility between the GNU "cp" tool and the Rust cp tool. While concerning I don't think that this is Postgres's fault.
I am compiling Postgres from commit c95ede41b8d47b21d58702fbc519e720f41fdaf1 on master. I am running "gmake clean && ./configure --prefix=$HOME/pq && gmake && gmake install" (gmake being GNU make). I receive the following error on either the "gmake install" or "gmake check" steps: