Pin home-manager version

nix/sources.json

@@ -1,4 +1,16 @@
 {
+    "home-manager": {
+        "branch": "release-20.09",
+        "description": "Manage a user environment using Nix  [maintainer=@rycee] ",
+        "homepage": "https://nix-community.github.io/home-manager/",
+        "owner": "nix-community",
+        "repo": "home-manager",
+        "rev": "63f299b3347aea183fc5088e4d6c4a193b334a41",
+        "sha256": "0iksjch94wfvyq0cgwv5wq52j0dc9cavm68wka3pahhdvjlxd3js",
+        "type": "tarball",
+        "url": "https://github.com/nix-community/home-manager/archive/63f299b3347aea183fc5088e4d6c4a193b334a41.tar.gz",
+        "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
+    },
     "krops": {
         "branch": "master",
         "description": "Mirror of https://cgit.krebsco.de/krops/about/ (PRs / issues accepted, as can be seen by not having them disabled)",

users/simon/default.nix

@@ -1,7 +1,7 @@
 { config, pkgs, ... }:
 {
   imports = [
-    (import "${builtins.fetchTarball "https://github.com/nix-community/home-manager/archive/release-20.09.tar.gz"}/nixos")
+    (import "${(import ../../nix/sources.nix).home-manager}/nixos")
   ];
 
   users.users.simon = {