Add the plugin install step to action (#26)

Author: choldgraf

.github/workflows/deploy.yml

@@ -36,6 +36,8 @@ jobs:
           node-version: 18.x
       - name: Install MyST Markdown
         run: npm install -g mystmd
+      - name: Install the local plugin
+        run: npm install
       - name: Build HTML Assets
         run: myst build --html
       - name: Upload artifact