Revert "bump nixpkgs to 8c809a1 (2026-03-03) — fixes matplotlib build"

This reverts commit 29a9de672e.

flake.lock

@@ -2,17 +2,17 @@
   "nodes": {
     "nixpkgs": {
       "locked": {
-        "lastModified": 1772542754,
-        "narHash": "sha256-WGV2hy+VIeQsYXpsLjdr4GvHv5eECMISX1zKLTedhdg=",
+        "lastModified": 1771848320,
+        "narHash": "sha256-0MAd+0mun3K/Ns8JATeHT1sX28faLII5hVLq0L3BdZU=",
         "owner": "NixOS",
         "repo": "nixpkgs",
-        "rev": "8c809a146a140c5c8806f13399592dbcb1bb5dc4",
+        "rev": "2fc6539b481e1d2569f25f8799236694180c0993",
         "type": "github"
       },
       "original": {
         "owner": "NixOS",
         "repo": "nixpkgs",
-        "rev": "8c809a146a140c5c8806f13399592dbcb1bb5dc4",
+        "rev": "2fc6539b481e1d2569f25f8799236694180c0993",
         "type": "github"
       }
     },
@@ -25,12 +25,6 @@
         "nixpkgs-bim": [
           "nixpkgs"
         ],
-        "nixpkgs-ci": [
-          "nixpkgs"
-        ],
-        "nixpkgs-emmett": [
-          "nixpkgs"
-        ],
         "nixpkgs-howard": [
           "nixpkgs"
         ],

flake.nix

@@ -3,7 +3,7 @@
 
   inputs = {
     # Default pin — all servers follow this unless overridden below
-    nixpkgs.url = "github:NixOS/nixpkgs/8c809a146a140c5c8806f13399592dbcb1bb5dc4";
+    nixpkgs.url = "github:NixOS/nixpkgs/2fc6539b481e1d2569f25f8799236694180c0993";
 
     # Per-server pins (follow default; replace with a direct url to override)
     nixpkgs-armer.follows = "nixpkgs";