Only install manrope if it's missing

Author: aomarks

packages/lit-dev-content/package.json

@@ -7,7 +7,7 @@
   "license": "BSD-3-Clause",
   "main": "_site/index.html",
   "scripts": {
-    "postinstall": "npm run fonts:manrope",
+    "postinstall": "ls site/fonts/manrope* > /dev/null || npm run fonts:manrope",
     "build": "rm -rf _site && npm run build:ts && npm run build:rollup && npm run build:samples && npm run build:site",
     "build:ts": "../../node_modules/.bin/tsc",
     "build:ts:watch": "../../node_modules/.bin/tsc --watch",