minimal-bootstrap.xz: parallelise

pkgs/os-specific/linux/minimal-bootstrap/default.nix

···
     xz = callPackage ./xz {
       bash = bash_2_05;
       tinycc = tinycc-musl;
+      gnumake = gnumake-musl;
+      gnutar = gnutar-musl;
     };
 
     inherit (callPackage ./utils.nix { }) derivationWithMeta writeTextFile writeText;

pkgs/os-specific/linux/minimal-bootstrap/xz/default.nix

···
     --disable-assembler
 
   # Build
-  make
+  make -j $NIX_BUILD_CORES
 
   # Install
-  make install
+  make -j $NIX_BUILD_CORES install
 ''