flake: split mediaserver onto nixos-25.11, keep desktops on unstable

The mediaserver kept hard-freezing on local builds (gnupg, openldap,
deno/rusty-v8) whenever a fresh unstable revision outran Hydra's
binary cache. It doesn't need bleeding-edge packages — every service
it runs is mature enough that 6-month-old versions are fine — so move
it onto the stable channel where the cache is essentially always
warm. Gaming and Macbook stay on unstable for fresh GPU/kernel work.

Implementation: add nixpkgs-stable + home-manager-stable inputs,
parameterise mkHost to accept a (nixpkgs, home-manager) pair.

Drive-by:
- Switch homepage.nix from environmentFiles (plural, unstable-only)
  to environmentFile (singular, present on both channels).
- Gate the openldap-skip-tests overlay to non-mediaserver hosts so
  it doesn't force a local rebuild on stable, where openldap is
  always cached.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

flake.lock

@@ -86,6 +86,27 @@
         "type": "github"
       }
     },
+    "home-manager-stable": {
+      "inputs": {
+        "nixpkgs": [
+          "nixpkgs-stable"
+        ]
+      },
+      "locked": {
+        "lastModified": 1775425411,
+        "narHash": "sha256-KY6HsebJHEe5nHOWP7ur09mb0drGxYSzE3rQxy62rJo=",
+        "owner": "nix-community",
+        "repo": "home-manager",
+        "rev": "0d02ec1d0a05f88ef9e74b516842900c41f0f2fe",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nix-community",
+        "ref": "release-25.11",
+        "repo": "home-manager",
+        "type": "github"
+      }
+    },
     "nix-cachyos-kernel": {
       "inputs": {
         "cachyos-kernel": "cachyos-kernel",
@@ -140,6 +161,22 @@
         "type": "github"
       }
     },
+    "nixpkgs-stable": {
+      "locked": {
+        "lastModified": 1777077449,
+        "narHash": "sha256-AIiMJiqvGrN4HyLEbKAoCSRRYn0rnlW5VbKNIMIYqm4=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "a4bf06618f0b5ee50f14ed8f0da77d34ecc19160",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixos-25.11",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
     "nixpkgs_2": {
       "locked": {
         "lastModified": 1777268161,
@@ -159,8 +196,10 @@
     "root": {
       "inputs": {
         "home-manager": "home-manager",
+        "home-manager-stable": "home-manager-stable",
         "nix-cachyos-kernel": "nix-cachyos-kernel",
         "nixpkgs": "nixpkgs_2",
+        "nixpkgs-stable": "nixpkgs-stable",
         "zen-browser": "zen-browser"
       }
     },