Revert "patch-shebangs: respect cross compilation"

This causes problems for packages built using a bootstrap stdenv,
resulting in references to /bin/sh or even bootstrap-tools. The darwin
stdenv is much stricter about what requisites/references are allowed but
using /bin/sh on linux is also undesirable.

eg. https://hydra.nixos.org/build/81754896

    $ nix-build -A xz
    $ head -n1 result-bin/bin/xzdiff
    #!/nix/store/yvc7kmw98kq547bnqn1afgyxm8mxdwhp-bootstrap-tools/bin/sh

This reverts commit f0694232.