Build fails on Windows.

`npm run build`

```
...
[13:21:21] Starting '_packageTemplates'...
Bundling: alp
Finished Bundling: alp
Bundling: themes_2
Finished Bundling: themes_2
C:\...\ampstart\node_modules\fs-extra\lib\mkdirs\mkdirs-sync.js:20
throw errInval
^

Error: C:\...\ampstart\.archive\alp\C:\Users\...\ampstart\dist\templates contains invalid WIN32 path characters.
```

Where `...` is my local path to the project `Users\******\Documnets\projects\`

Start by running `npm run build` on Windows and follow the `_packageTemplates` step that reports the invalid WIN32 path. Investigate the path handling around the `.archive` and `dist\templates` locations; done means the build completes successfully on Windows without the invalid-path error.

Written by the indexing model from the issue text.