add install pnpm

Author: wechulisimiyu

.github/workflows/nextjs.yml

@@ -78,6 +78,9 @@ jobs:
             ${{ runner.os }}-nextjs-${{ hashFiles('**/package-lock.json', '**/yarn.lock') }}-
       - name: Install dependencies
         run: ${{ steps.detect-package-manager.outputs.manager }} ${{ steps.detect-package-manager.outputs.command }}
+      - name: Install pnpm
+        if: steps.detect-package-manager.outputs.manager == 'pnpm'
+        run: npm install -g pnpm
       - name: Build with Next.js
         run: ${{ steps.detect-package-manager.outputs.runner }} next build
       - name: Upload artifact