chore: use concurrently to run parallel builds (#3785)

## About the changes
Small improvement to speed up the build, tested locally validating the
output with and without concurrently.
**Note** this also removes copy-templates from the backend build

package.json

@@ -31,16 +31,16 @@
   "main": "./dist/lib/server-impl.js",
   "scripts": {
     "start": "TZ=UTC node ./dist/server.js",
-    "build:backend": "copyfiles -u 1 src/mailtemplates/**/*.mustache dist/ && tsc --pretty --strictNullChecks false",
+    "copy-templates": "copyfiles -u 1 src/mailtemplates/**/*.mustache dist/",
+    "build:backend": "tsc --pretty --strictNullChecks false",
     "build:frontend": "yarn --cwd ./frontend run build",
-    "build": "yarn run build:frontend && yarn run build:backend",
+    "build": "concurrently \"yarn:copy-templates\" \"yarn:build:frontend\" \"yarn:build:backend\"",
     "dev:backend": "TZ=UTC NODE_ENV=development tsc-watch --strictNullChecks false --onSuccess \"node dist/server-dev.js\"",
     "dev:frontend": "wait-on tcp:4242 && yarn --cwd ./frontend run dev",
     "dev": "concurrently \"yarn:dev:backend\" \"yarn:dev:frontend\"",
     "prepare": "node scripts/husky-install && yarn --cwd ./frontend install && if [ ! -d ./dist ]; then yarn build; fi",
     "prestart:dev": "yarn run clean",
     "start:dev": "TZ=UTC NODE_ENV=development tsc-watch --strictNullChecks false --onSuccess \"node dist/server-dev.js\"",
-    "copy-templates": "copyfiles -u 1 src/mailtemplates/**/*.mustache dist/",
     "db-migrate": "db-migrate --migrations-dir ./src/migrations",
     "lint": "eslint ./src",
     "local:package": "del-cli --force build && mkdir build && cp -r dist docs CHANGELOG.md LICENSE README.md package.json build",