Nix-dev-env: update GHC to current Nixpkgs default

Author: Anton-Latukha

default.nix

@@ -1,7 +1,7 @@
 {
 # For current default and explicitly supported GHCs https://search.nixos.org/packages?query=ghc&from=0&size=500&channel=unstable, Nixpkgs implicitly supports older minor versions also, until the configuration departs from compatibility with them.
 # Compiler in a form ghc8101 <- GHC 8.10.1, just remove spaces and dots
-  compiler    ? "ghc8104"
+  compiler    ? "ghc8107"
 
 # Deafult.nix is a unit package abstraciton that allows to abstract over packages even in monorepos:
 # Example: pass --arg cabalName --arg packageRoot "./subprojectDir", or map default.nix over a list of tiples for subprojects.

shell.nix

@@ -2,6 +2,6 @@ attrs@{...}:
 let defaultAttrs = {
   # Defaults are put in this form deliberately. Details: #748
   withHoogle = true;
-  compiler = "ghc8104";
+  compiler = "ghc8107";
 };
 in (import ./. (defaultAttrs // attrs)).env