♻️(mail) refactor mail to use npm instead of yarn

Following @manuhabitela's recommendation, we've decided to switch
from Yarn to npm for managing our Node.js dependencies. This update
aligns all remaining parts of the codebase that were still using
Yarn to now utilize npm.

src/mail/package.json

@@ -11,7 +11,7 @@
   "scripts": {
     "build-mjml-to-html": "bash ./bin/mjml-to-html",
     "build-html-to-plain-text": "bash ./bin/html-to-plain-text",
-    "build": "yarn build-mjml-to-html && yarn build-html-to-plain-text"
+    "build": "npm run build-mjml-to-html && npm run build-html-to-plain-text"
   },
   "volta": {
     "node": "16.15.1"