Update preview command for ts sdk to use pnpm

Author: fern-support

.github/workflows/preview_sdks.yml

@@ -30,8 +30,8 @@ jobs:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
         run: |
           cd fern/.preview/fern-typescript-sdk
-          yarn install
-          yarn build
+          pnpm install
+          pnpm build
 
   preview-java:
     runs-on: ubuntu-latest