kexec: reduce closure size

The tarball was referring to ncurses, icu and glibc, which blow up the
image to 1.4-1.5GB. With these changes the tarball is shrinked back to
~700MB.

Author: Mic92

kexec/kexec.nix

@@ -13,8 +13,8 @@
       executable = true;
       name = "kexec-nixos";
       text = ''
-        #!${pkgs.stdenv.shell}
-        export PATH=${pkgs.kexectools}/bin:${pkgs.cpio}/bin:$PATH
+        #!${pkgs.pkgsMusl.busybox}/bin/sh
+        export PATH=${pkgs.pkgsMusl.kexectools}/bin:${pkgs.pkgsMusl.busybox}/bin
         set -x
         set -e
         cd $(mktemp -d)
@@ -39,10 +39,12 @@
     mkdir -p /mnt-root/root/.ssh/
     cp /authorized_keys /mnt-root/root/.ssh/
   '';
-  system.build.kexec_tarball = pkgs.callPackage <nixpkgs/nixos/lib/make-system-tarball.nix> {
+  system.build.kexec_tarball = pkgs.callPackage (pkgs.path + "/nixos/lib/make-system-tarball.nix") {
     storeContents = [
       { object = config.system.build.kexec_script; symlink = "/kexec_nixos"; }
     ];
-    contents = [];
+    contents = [
+      { source = image; target = image; }
+    ];
   };
 }