switch-to-configuration: use absolute path to systemd-escape

Fixes issue when upgrading from very old NixOS systems that don't have
systemd-escape in $PATH:

  $ sudo nixos-rebuild switch
  ...
  building the system configuration...
  updating GRUB 2 menu...
  Can't exec "systemd-escape": No such file or directory at /nix/var/nix/profiles/system/bin/switch-to-configuration line 264.
  Unable to escape /!