Fix packing nixexprs in release job, better compression

upstream introduced a test which tests behaviour with broken symlinks,
related to RFC 140.
Our combined sources for the release tarball contain symlinks to the
various inputs (nixpkgs, fc, nixos-mailserver) which have to be
dereferenced by tar, thus using the -h option. This causes tar to
fail on the test symlink which points to nothing.

We use tar now without dereferencing which makes it a bit more
complicated to get the desired archive structure.

This change also modifies xz compression settings to reduce file size
and decompression time. xz now uses the default of -6 with additional
"extreme" tuning which reduces size even further without increasing
decompression time. We now also use all available cores for compression.

release/default.nix

@@ -333,17 +333,27 @@ jobs // {
 
     patchPhase = "touch .update-on-nixos-rebuild";
 
-    XZ_OPT = "-1";
     tarOpts = ''
-      --owner=0 --group=0 --mtime="1970-01-01 00:00:00 UTC" \
+      --owner=0 --group=0 \
+      --mtime="1970-01-01 00:00:00 UTC" \
       --exclude-vcs-ignores \
-      --transform='s!^\.!${name}!' \
     '';
 
     installPhase = ''
       mkdir -p $out/{tarballs,nix-support}
-      cd nixos
-      tar cJhf $out/tarballs/nixexprs.tar.xz ${tarOpts} .
+      tarball=$out/tarballs/nixexprs.tar
+
+      for file in nixos/* nixos/.*; do
+          if [ -f "$file" ]; then
+              tar uf "$tarball" --transform "s|^nixos|${name}|" ${tarOpts} "$file"
+          fi
+      done
+
+      for d in nixos/*/; do
+          tar uf "$tarball" --transform "s|^$d\\.|${name}/$(basename "$d")|" ${tarOpts} "$d."
+      done
+
+      xz -T0 -e "$tarball"
 
       echo "channel - $out/tarballs/nixexprs.tar.xz" > "$out/nix-support/hydra-build-products"
       echo $constituents > "$out/nix-support/hydra-aggregate-constituents"