chore: update flake.lock; add gnupg as dependency (#12497)

Add `gnupg` as part of the Nix-based development environment, which is a dependency for a small number of integration tests like `TestInstanceSigning`.  Bumps `flake.lock` from its current 2025-11-12 to current 2026-05-05 pin, bringing updated tools referenced in `shell.nix`.

```
$ nix flake update
• Updated input 'nixpkgs':
    'github:nixos/nixpkgs/c5ae371f1a6a7fd27823bc500d9390b38c05fa55?narHash=sha256-4PqRErxfe%2B2toFJFgcRKZ0UI9NSIOJa%2B7RXVtBhy4KE%3D' (2025-11-12)
  → 'github:nixos/nixpkgs/549bd84d6279f9852cae6225e372cc67fb91a4c1?narHash=sha256-hGdgeU2Nk87RAuZyYjyDjFL6LK7dAZN5RE9%2BhrDTkDU%3D' (2026-05-05)
```

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### Documentation

- [ ] I created a pull request [to the documentation](https://codeberg.org/forgejo/docs) to explain to Forgejo users how to use this change.
- [x] I did not document these changes and I do not expect someone else to do it.

### Release notes

- [ ] This change will be noticed by a Forgejo user or admin (feature, bug fix, performance, etc.). I suggest to include a release note for this change.
- [x] This change is not visible to a Forgejo user or admin (refactor, dependency upgrade, etc.). I think there is no need to add a release note for this change.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/12497
Reviewed-by: Andreas Ahlenstorf <aahlenst@noreply.codeberg.org>

flake.lock

@@ -2,11 +2,11 @@
   "nodes": {
     "nixpkgs": {
       "locked": {
-        "lastModified": 1762977756,
-        "narHash": "sha256-4PqRErxfe+2toFJFgcRKZ0UI9NSIOJa+7RXVtBhy4KE=",
+        "lastModified": 1777954456,
+        "narHash": "sha256-hGdgeU2Nk87RAuZyYjyDjFL6LK7dAZN5RE9+hrDTkDU=",
         "owner": "nixos",
         "repo": "nixpkgs",
-        "rev": "c5ae371f1a6a7fd27823bc500d9390b38c05fa55",
+        "rev": "549bd84d6279f9852cae6225e372cc67fb91a4c1",
         "type": "github"
       },
       "original": {

shell.nix

@@ -25,5 +25,6 @@ pkgs.mkShell {
 
     # tests
     openssh
+    gnupg
   ];
 }