Move postInstall script to a shell file

package.json

@@ -7,7 +7,7 @@
   "license": "MIT",
   "private": true,
   "scripts": {
-    "postinstall": "npx patch-package && cd desktop && npm install",
+    "postinstall": "scripts/postInstall.sh",
     "clean": "npx react-native clean-project-auto",
     "android": "scripts/set-pusher-suffix.sh && npx react-native run-android --port=8083",
     "ios": "scripts/set-pusher-suffix.sh && npx react-native run-ios --port=8082",

scripts/postInstall.sh

@@ -0,0 +1,16 @@
+#!/bin/bash
+
+# Go to project root
+ROOT_DIR=$(dirname "$(cd -- "$(dirname -- "${BASH_SOURCE[0]}")" &> /dev/null && pwd)")
+cd "$ROOT_DIR" || exit 1
+
+# Run patch-package
+npx patch-package
+
+# Install node_modules in subpackages, unless we're in a CI/CD environment.
+# See `.github/actions/composite/setupNode/action.yml` for more context
+if [[ -n ${CI+x} ]]; then
+  echo 'Installing desktop/node_modules'
+  cd desktop || exit 1
+  npm install
+fi