base: nix: nix 2.4 is now stable

Flake lock file changes:

• Updated input 'emacs-overlay':
    'github:nix-community/emacs-overlay/086344663fbd821023dd4cce8cb5eecc502c2769' (2021-11-08)
  → 'github:nix-community/emacs-overlay/cc2cdfc1212057526f1f8f48b91cf58f49a1c3ae' (2021-11-19)
• Updated input 'home-manager':
    'github:nix-community/home-manager/2917ef23b398a22ee33fb34b5766b28728228ab1' (2021-11-07)
  → 'github:nix-community/home-manager/2452979efe92128b03e3c27567267066c2825fab' (2021-11-18)
• Updated input 'nixpkgs':
    'github:NixOS/nixpkgs/c935f5e0add2cf0ae650d072c8357533e21b0c35' (2021-11-07)
  → 'github:NixOS/nixpkgs/931ab058daa7e4cd539533963f95e2bb0dbd41e6' (2021-11-15)
• Updated input 'nixpkgs-unstable-small':
    'github:NixOS/nixpkgs/c9acf4782f2a830da29c18b77ec564cb73e22946' (2021-11-07)
  → 'github:NixOS/nixpkgs/42d32516400c1d821d275a5460900bbaef3d3bf1' (2021-11-18)

base/nix.nix

@@ -3,7 +3,7 @@
   nixpkgs.config.allowUnfree = true;
 
   nix = {
-    package = pkgs.nixUnstable;
+    package = pkgs.nixStable;
     extraOptions = ''
       experimental-features = nix-command flakes
     '';

flake.lock

@@ -2,11 +2,11 @@
   "nodes": {
     "emacs-overlay": {
       "locked": {
-        "lastModified": 1636363962,
-        "narHash": "sha256-DyF+NBIPF7f8IZ6zI0NKbTjnfAq7tTSr74k2zVolQsw=",
+        "lastModified": 1637314118,
+        "narHash": "sha256-npGYpjp2Zc53p3adMQouTq+F+YI/X1aJiDL0DozzWOs=",
         "owner": "nix-community",
         "repo": "emacs-overlay",
-        "rev": "086344663fbd821023dd4cce8cb5eecc502c2769",
+        "rev": "cc2cdfc1212057526f1f8f48b91cf58f49a1c3ae",
         "type": "github"
       },
       "original": {
@@ -39,11 +39,11 @@
         ]
       },
       "locked": {
-        "lastModified": 1636274622,
-        "narHash": "sha256-tZYuGhqcfH7piCsrUrIYM0P3oPJcoBxGkuxeFNVxkCc=",
+        "lastModified": 1637249535,
+        "narHash": "sha256-RCatEYQ+uqsZOZpN4ZOtSoO7CJTiQpHNdPjUA0jtejw=",
         "owner": "nix-community",
         "repo": "home-manager",
-        "rev": "2917ef23b398a22ee33fb34b5766b28728228ab1",
+        "rev": "2452979efe92128b03e3c27567267066c2825fab",
         "type": "github"
       },
       "original": {
@@ -71,11 +71,11 @@
     },
     "nixpkgs": {
       "locked": {
-        "lastModified": 1636267212,
-        "narHash": "sha256-KDS173KqmqrYUPY9N4vf750GxIo+S6E0djyq2BsQm8s=",
+        "lastModified": 1636976544,
+        "narHash": "sha256-9ZmdyoRz4Qu8bP5BKR1T10YbzcB9nvCeQjOEw2cRKR0=",
         "owner": "NixOS",
         "repo": "nixpkgs",
-        "rev": "c935f5e0add2cf0ae650d072c8357533e21b0c35",
+        "rev": "931ab058daa7e4cd539533963f95e2bb0dbd41e6",
         "type": "github"
       },
       "original": {
@@ -87,11 +87,11 @@
     },
     "nixpkgs-unstable-small": {
       "locked": {
-        "lastModified": 1636300541,
-        "narHash": "sha256-UiaOhzCeJX0EpHl/iVxsqpoS0blSG2yZnR4JrN8GcvM=",
+        "lastModified": 1637209424,
+        "narHash": "sha256-oXw75hkCOVtoB+CEElWiTmkC1gNdL3jf0tG2GInytHA=",
         "owner": "NixOS",
         "repo": "nixpkgs",
-        "rev": "c9acf4782f2a830da29c18b77ec564cb73e22946",
+        "rev": "42d32516400c1d821d275a5460900bbaef3d3bf1",
         "type": "github"
       },
       "original": {