test: pin netlify-cli to 17.6.0 (#724)

Author: pieh

.github/workflows/test.yml

@@ -29,7 +29,7 @@ jobs:
           key:
             ubuntu-build-${{ env.cache-name }}-${{
             hashFiles('plugin/test/fixtures/**/package.json') }}-node-modules
-      - run: npm install -g netlify-cli
+      - run: npm install -g netlify-cli@17.6.0
       - run: npm ci
       - run: cd plugin && npm ci && npm run build
       - run: npm test