Rust nightly of 2018-12-08 fails

[lovesegfault]

Trying to run a shell with the 2018-12-08 nightly I get:

$ nix-shell --pure --show-trace
warning: Nix search path entry '/nix/var/nix/profiles/per-user/root/channels' does not exist, ignoring
warning: Nix search path entry '/nix/var/nix/profiles/per-user/root/channels/nixpkgs' does not exist, ignoring
error: while evaluating the attribute 'buildInputs' of the derivation 'sensord' at /nix/store/pacphcqmamrz5z0mnvxvkbbz0hdkn038-source/pkgs/stdenv/generic/make-derivation.nix:179:11:
while evaluating the attribute 'paths' of the derivation 'rust-1.32.0-nightly-2018-12-07-4a45578bc' at /nix/store/pacphcqmamrz5z0mnvxvkbbz0hdkn038-source/pkgs/stdenv/generic/make-derivation.nix:179:11:
while evaluating the attribute 'src' of the derivation 'rls-preview' at /nix/store/pacphcqmamrz5z0mnvxvkbbz0hdkn038-source/pkgs/stdenv/generic/make-derivation.nix:179:11:
while evaluating the derivation attribute 'name' at /nix/store/pacphcqmamrz5z0mnvxvkbbz0hdkn038-source/pkgs/stdenv/generic/make-derivation.nix:179:11:
attribute 'xz_url' missing, at /nix/store/vsvxy129sl9bg1cwmsrdjjngj1w796x1-source/rust-overlay.nix:100:31

[bkchr]

This is a current limitation of our download implementation. There was just no nightly in the 2018-12-08. We need to fix this by providing a way to fallback to the latest available release.

[florianjacob]

Related: #108 , but with extensions instead of the full release.