glibc's libcrypt is deprecated and since
ff30c899d8
is built by default without libcrypt, that's probably the point when `elasticsearch6`
started failing with
```
auto-patchelf: 3 dependencies could not be satisfied
error: auto-patchelf could not satisfy dependency libcrypt.so.1 wanted by /nix/store/nd0gn95yfnnmnnw8zk2jnafc9gj2qy91-elasticsearch-6.8.21/modules/x-pack-ml/platform/linux-x86_64/lib/liblog4cxx.so.10
error: auto-patchelf could not satisfy dependency libcrypt.so.1 wanted by /nix/store/nd0gn95yfnnmnnw8zk2jnafc9gj2qy91-elasticsearch-6.8.21/modules/x-pack-ml/platform/linux-x86_64/lib/libaprutil-1.so.0
error: auto-patchelf could not satisfy dependency libcrypt.so.1 wanted by /nix/store/nd0gn95yfnnmnnw8zk2jnafc9gj2qy91-elasticsearch-6.8.21/modules/x-pack-ml/platform/linux-x86_64/lib/libapr-1.so.0
```
Let's add libxcrypt dependency, also note that `elasticsearch6-oss` doesn't
seem to need it.
Should resolve https://github.com/NixOS/nixpkgs/issues/203467
Extra note is elk6 may get removed from nixpkgs soon in favor of elk7
https://github.com/NixOS/nixpkgs/pull/194420
18271606a2