Added yarn build command

refs https://github.com/TryGhost/Toolbox/issues/390

- this allows us to run `yarn build` and make it output a .tgz file
  which can be installed with `ghost install --archive ...`

ghost/core/package.json

@@ -22,7 +22,7 @@
   "scripts": {
     "start": "node index",
     "setup": "knex-migrator init && grunt symlink || (exit 0)",
-    "build": "yarn workspace ghost-admin run build",
+    "build": "npm pack --pack-destination ../..",
     "test": "yarn test:unit",
     "test:single": "mocha --require=./test/utils/overrides.js --exit --trace-warnings --recursive --extension=test.js --timeout=60000",
     "test:all": "yarn test:unit && yarn test:integration && yarn test:e2e && yarn lint",

package.json

@@ -19,6 +19,7 @@
     "**/node_modules/**"
   ],
   "scripts": {
+    "build": "yarn workspace ghost run build",
     "dev:debug": "DEBUG_COLORS=true DEBUG=@tryghost*,ghost:* yarn dev",
     "dev:admin": "node .github/dev.js --admin",
     "dev:ghost": "node .github/dev.js --ghost",