Handle building from a dirty Git tree

Dependencies

• [2] JUQJHSYJ release.nix: Add `--with-docbook-xsl' to the configure flags for `tarball'.
• [*] T4LLYESZ * Nix expression for building Hydra.
• [*] OEKXEMU2 hydra: Change the `tarball' job to use `--with-nix'.

Change contents

• edit in release.nix at line 53
  [5.1]

  [2.0]
  + postUnpack = ''

  + # Clean up when building from a working tree.

  + if [ -z "$IN_NIX_SHELL" ]; then

  + (cd $sourceRoot && (git ls-files -o --directory | xargs -r rm -rfv)) || true

  + fi

  + '';

  +