use yarn in release-plan CI

Author: mansona

.github/workflows/plan-release.yml

@@ -50,7 +50,7 @@ jobs:
         with:
           node-version: 18
 
-      - run: npm ci
+      - run: yarn install --frozen-lockfile
 
       - name: "Generate Explanation and Prep Changelogs"
         id: explanation

.github/workflows/publish.yml

@@ -47,7 +47,7 @@ jobs:
           # This creates an .npmrc that reads the NODE_AUTH_TOKEN environment variable
           registry-url: 'https://registry.npmjs.org'
 
-      - run: npm ci
+      - run: yarn install --frozen-lockfile
       - name: npm publish
         run: npx release-plan publish