Use rsync --chmod=u+w to fix builds on VirtioFS-backed stores
Apple's VirtioFS (used by nix-linux-builder for Linux-on-Darwin builds) does not honor Linux DAC_OVERRIDE. When rsync -a copies from read-only Nix store paths, it preserves the 555 directory permissions, creating truly unwritable directories on VirtioFS. Subsequent file writes into these directories fail with EACCES. Adding --chmod=u+w to rsync makes directories owner-writable as they are created, preventing the permission lockout. This is a no-op on native Linux where DAC_OVERRIDE already grants write access regardless of directory permissions. The separate chmod -R +w calls are no longer needed since rsync now sets the correct permissions during copy.
