chore: create shell.nix and update flake.* (#8129)

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/8129
Reviewed-by: Gusted <gusted@noreply.codeberg.org>
Co-authored-by: famfo <famfo@famfo.xyz>
Co-committed-by: famfo <famfo@famfo.xyz>

shell.nix

@@ -0,0 +1,28 @@
+{
+  pkgs ? import <nixpkgs> { },
+}:
+
+pkgs.mkShell {
+  name = "forgejo";
+  nativeBuildInputs = with pkgs; [
+    # generic
+    git
+    git-lfs
+    gnumake
+    gnused
+    gnutar
+    gzip
+
+    # frontend
+    nodejs
+
+    # backend
+    gofumpt
+    sqlite
+    go
+    gopls
+
+    # tests
+    openssh
+  ];
+}