xen: fix script shebangs (#437892)

Changed files
+2
pkgs
by-name
xe
+2
pkgs/by-name/xe/xen/package.nix
···
pandoc,
# Scripts
+
bash,
bridge-utils,
coreutils,
diffutils,
···
]);
buildInputs = [
+
bash
bzip2
e2fsprogs.dev
libnl