Drop the invalid --prefer-offline flag from composer install

`--prefer-offline` is an npm flag; composer install has no such option and
exits 1 on it, so the build failed before installing anything. It was
copy-pasted from the `npm ci --prefer-offline` line three lines below,
in 9bed611.

Composer uses its cache automatically and --prefer-dist is already
present, so there is nothing to replace the flag with.

This breaks every gha-build run on develop, not just this branch.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019LoLgW7oFjBPxGea9kiZJ2

Author: pattonwebz

.github/workflows/build-plugin-with-ref.yml

@@ -107,7 +107,7 @@ jobs:
 
       - name: Install Composer dependencies
         run: |
-          if [ -f composer.json ]; then composer install --no-dev --prefer-dist --prefer-offline --no-progress --no-interaction; else echo "No composer.json"; fi
+          if [ -f composer.json ]; then composer install --no-dev --prefer-dist --no-progress --no-interaction; else echo "No composer.json"; fi
 
       - name: Install npm dependencies
         if: steps.cache-node-modules.outputs.cache-hit != 'true' && hashFiles('package.json') != ''