Fix simple-shell.nix script

[mpickering]

The simple-shell.nix script attempts to create a shell using a prebuilt artifact. Unfortunately it doesn't work and I don't know why.

<command line>: /nix/store/iykxb0bmfjmi7s53kfg6pjbfpd8jmza6-glibc-2.27/lib/librt.so: symbol __close_nocancel, version GLIBC_PRIVATE not defined in file libc.so.6 with link time reference
cabal: Failed to build dwarfadt-0.6 (which is required by exe:debugger from
ghc-debugger-0.1.0.0, exe:dumpdwarf from dwarfadt-0.6 and others).

[mpickering]

I fixed it by pinning to an older version of nixpkgs for now.

[supersven]

Currently simple-shell.nix doesn't work at all, because gitlab.haskell.org artifacts are removed after 3 months.

If someone wants to join and can't because of this: Please drop me a note and I'll upload binaries somewhere else.