systemd: set kexec-path meson option

Since systemd/systemd@c3f32b9, meson no longer probes $PATH to
locate kexec and instead hardcodes /usr/bin/kexec unless the
-Dkexec-path option is explicitly set.

kexec has probably only ever worked on NixOS because of the
fallback codepath in shutdown.c that reboots via RB_KEXEC when
the kexec binary isn't found.

Fixes #498149

pkgs/os-specific/linux/systemd/default.nix

@@ -560,6 +560,9 @@ stdenv.mkDerivation (finalAttrs: {
     (lib.mesonOption "loadkeys-path" "${kbd}/bin/loadkeys")
     (lib.mesonOption "setfont-path" "${kbd}/bin/setfont")
   ]
+  ++ lib.optionals withKexectools [
+    (lib.mesonOption "kexec-path" "${kexec-tools}/bin/kexec")
+  ]
   ++ lib.optionals withKmod [
     (lib.mesonOption "kmod-path" "${kmod}/bin/kmod")
   ]