Make sources.nix generation compatible with nixpkgs-fmt

[lovesegfault]

It almost is, the only changes it tries to apply are these:

diff --git a/nix/sources.nix b/nix/sources.nix
index 8a725cb..5b27300 100644
--- a/nix/sources.nix
+++ b/nix/sources.nix
@@ -1,7 +1,5 @@
 # This file has been generated by Niv.
-
 let
-
   #
   # The fetchers. fetch_<type> fetches specs of type <type>.
   #

[zimbatm]

note that nixpkgs-fmt isn't stable yet, the format is still evolving until the 1.0 is released.

[nmattia]

@zimbatm is there an ETA for 1.0? @lovesegfault we're currently nixpkgs-fmt-ing it, but with an older version...

[zimbatm]

the ETA was last week :) we are running into complications with the core engine. giving it 2 more weeks from now.

[nmattia]

Hey @lovesegfault, would love your feedback on #272. The --light template should alleviate the issue.