nixos/sysctl: rename /etc/sysctl.d/nixos.conf -> 60-nixos.conf

sysctl.d(5) recommends prefixing all filenames in /etc/sysctl.d with a
two-digit number and a dash, to simplify the ordering of the files.

Some packages provide custom files, often with "50-" prefix.
To ensure user-supplied configuration takes precedence over the one
specified via `boot.kernel.sysctl`, prefix the file generated there with
"60-".