fix(ci): install fish in build-docs workflow

The registry round-trip test spawns a real fish subprocess to verify
conf.d/__fish_config_op_registry.fish sources correctly, but the
runner image has no fish binary, so the workflow failed with
FileNotFoundError.

.github/workflows/build-docs.yml

@@ -27,7 +27,7 @@ jobs:
       - name: Install dependencies
         run: |
           sudo apt-get update -qq
-          sudo apt-get install -y pandoc python3-yaml
+          sudo apt-get install -y pandoc python3-yaml fish
 
       - name: Generate concatenated markdown
         run: python3 docs/build-manual.py --concat -o docs/fish-config.md