🐛 Fixed ghost.min.css asset compilation

refs https://github.com/TryGhost/DevOps/issues/48

- due to changes in the build workflow, we no longer depended on
  `ghost`'s own `build` before producing the tarball
- as a result, the minified css was not produced and private pages were
  unstyled
- this adds our own `build` step to the list of dependencies for
  `archive`

ghost/core/package.json

@@ -265,6 +265,7 @@
     "targets": {
       "archive": {
         "dependsOn": [
+          "build",
           "^build:ts",
           {
             "projects": [