Fix legacy caching module distribution for CI

The legacy caching bundle at sources/legacy-caching/dist/ was
gitignored by sources/.gitignore, so CI never included it. Fix by:
- Copying the bundle into sources/dist/legacy-caching/ during build
- Updating the loader path to resolve from dist/legacy-caching/
- Adding legacy module build steps to build-dist action and
  ci-update-dist workflow

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.github/workflows/ci-update-dist.yml

@@ -38,6 +38,12 @@ jobs:
       run: |
         npm exec -y -- pacote extract @gradle-tech/develocity-agent@3.0.1 ~/.node_libraries/@gradle-tech/develocity-agent
 
+    - name: Build legacy caching module
+      run: |
+        npm clean-install
+        npm run build
+      working-directory: sources/legacy-caching
+
     - name: Install npm dependencies
       run: |
         npm clean-install
@@ -58,6 +64,8 @@ jobs:
 
     - name: Copy the generated sources/dist directory to the top-level dist
       run: |
+        mkdir -p sources/dist/legacy-caching
+        cp sources/legacy-caching/dist/index.js sources/dist/legacy-caching/
         cp -r sources/dist .
 
     - name: Import GPG key to sign commits