auspice/_config.yml: exclude "vendor/"

[joverlee521]

GitHub Action ruby/setup-ruby does not allow customization of the vendor path¹ so just exclude the hard-coded vendor/ path in the config.

¹ ruby/setup-ruby#136

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This is required for setting up the GitHub Action workflow to deploy the private builds to netlify. Without this change, we run into errors when trying to build the site with jekyll:

ERROR: YOUR SITE COULD NOT BE BUILT:
------------------------------------
Invalid date '<%= Time.now.strftime('%Y-%m-%d %H:%M:%S %z') %>': Document 'vendor/bundle/ruby/2.3.0/gems/jekyll-3.1.2/lib/site_template/_posts/0000-00-00-welcome-to-jekyll.markdown.erb' does not have a valid date in the YAML front matter.