Nixfmt (#427)

## Description

Add nixfmt to shell.nix so we can check its formatting.

## Why is this needed

Like any fmt-esque tool, its output may differ between different revisions. So let's guard against that.

## How Has This Been Tested?

CI.


## How are existing users impacted? What migration steps/scripts do we need?

N/A

ci-checks.sh

@@ -22,4 +22,12 @@ if ! git ls-files '*.sh' | xargs shellcheck; then
 	failed=1
 fi
 
+if ! nixfmt shell.nix; then
+	failed=1
+fi
+
+if ! git diff | (! grep .); then
+	failed=1
+fi
+
 exit "$failed"

shell.nix

@@ -1,15 +1,11 @@
 let _pkgs = import <nixpkgs> { };
-in
-{ pkgs ?
-  import
-    (_pkgs.fetchFromGitHub {
-      owner = "NixOS";
-      repo = "nixpkgs-channels";
-      #branch@date: nixpkgs-unstable@2020-09-11
-      rev = "6d4b93323e7f78121f8d6db6c59f3889aa1dd931";
-      sha256 = "0g2j41cx2w2an5d9kkqvgmada7ssdxqz1zvjd7hi5vif8ag0v5la";
-    }) { }
-}:
+in { pkgs ? import (_pkgs.fetchFromGitHub {
+  owner = "NixOS";
+  repo = "nixpkgs-channels";
+  #branch@date: nixpkgs-unstable@2020-09-11
+  rev = "6d4b93323e7f78121f8d6db6c59f3889aa1dd931";
+  sha256 = "0g2j41cx2w2an5d9kkqvgmada7ssdxqz1zvjd7hi5vif8ag0v5la";
+}) { } }:
 
 with pkgs;
 
@@ -21,6 +17,7 @@ mkShell {
     go
     gotools
     jq
+    nixfmt
     nodePackages.prettier
     protobuf
     protoc-gen-doc