use npm@8 for test:fuzz

Author: gfx

.github/workflows/fuzz.yml

@@ -21,6 +21,7 @@ jobs:
         cache: npm
         node-version: "18"
 
-    - run: npm install -g npm
+    # npm@9 may fail with https://github.com/npm/cli/issues/6723
+    - run: npm install -g npm@8
     - run: npm ci
     - run: npm run test:fuzz