fix(devshell): work around godoc not supporting riscv5

nix/devshells/docs.nix

@@ -5,21 +5,23 @@
 }:
 pkgs.mkShellNoCC {
   packages = with pkgs;
-    [
-      perSystem.godoc.default
-      (pkgs.writeScriptBin "gen-reference" ''
-        out="./docs/content/reference/go_doc"
-        godoc -c -o $out .
-      '')
-      (pkgs.writeScriptBin "mkdocs" ''
-        # generate reference docs first
-        gen-reference
-        # execute the underlying command
-        ${pkgs.mkdocs}/bin/mkdocs "$@"
-      '')
-    ]
-    ++ (with pkgs.python3Packages; [
-      mike
-      mkdocs-material
-    ]);
+  # Pop an empty shell on systems that aren't supported by godoc
+    lib.optionals (perSystem.godoc ? default)
+    ([
+        perSystem.godoc.default
+        (pkgs.writeScriptBin "gen-reference" ''
+          out="./docs/content/reference/go_doc"
+          godoc -c -o $out .
+        '')
+        (pkgs.writeScriptBin "mkdocs" ''
+          # generate reference docs first
+          gen-reference
+          # execute the underlying command
+          ${pkgs.mkdocs}/bin/mkdocs "$@"
+        '')
+      ]
+      ++ (with pkgs.python3Packages; [
+        mike
+        mkdocs-material
+      ]));
 }