Add 'git checkout upstream/main' + pnpm install to JS automation to avoid pnpm-lock.yaml conflicts

Per https://github.com/Azure/azure-sdk-tools/issues/14540#issuecomment-4178843881:

In JS SDK automation, prior to running the build, add this step to ensure the pnpm-lock.yaml is up-to-date and avoids conflicts:

```
git checkout upstream/main "$(git rev-parse --show-toplevel)/pnpm-lock.yaml" && pnpm install
```

This should be included before the `pnpm build` step in the JS automation script to prevent issues with outdated lock files causing conflicts during SDK PRs.

Reference: [Root Issue #14540](https://github.com/Azure/azure-sdk-tools/issues/14540)

Locate the JS SDK automation script and its existing pnpm build step, using root issue #14540 for context. Add the requested lockfile checkout and pnpm install step immediately before pnpm build, then verify the automation runs with the step in that order and avoids outdated pnpm-lock.yaml conflicts.

Written by the indexing model from the issue text.