Update rollup to fix -actions build

[tomalec]

Changes proposed in this Pull Request:

Currently, github actions build fails
due to not supported nullish coalescing assignment

Unexpected token '??='
npm ERR! code ELIFECYCLE
npm ERR! errno 1
npm ERR! woocommerce-grow-github-actions@1.10.1 build: `npx rollup -c`

This PR updates the rollup, and adds it explicitly as devDependnecy

I'm not updating to the latest 4.x, as it requires Node >18

Screenshots:

Detailed test instructions:

  cd ./packages/github-actions
  npm ci --ignore-scripts
  npm run build

Additional details:

Changelog entry

Fix - Github Actions build, update rollup to 3.29.4.

[eason9487]

LGTM. Thanks for fixing the build processing!