Fix pnpm installation error on CI

Per <pnpm/action-setup#135>.

Author: caleb531

.github/workflows/tests.yml

@@ -22,7 +22,7 @@ jobs:
       - uses: actions/checkout@v3
 
       - name: Install PNPM
-        uses: pnpm/action-setup@v2
+        uses: pnpm/action-setup@v4
         with:
           version: latest