Assuming you have the Github CLI installed.
git clone https://github.com/n8n-io/n8n.git
cd n8n
gh pr checkout fix/mailjet-node
lerna bootstrap --hoist
npm run build
npm run dev
After that, it should be available in localhost.
In case you do not have installed lerna, npm, etc, you can learn how to do with the link below:
Let me know if that works.