Fixed sitemap.xml uses localhost instead of https://helm.sh

[Andygol]

Changed SITE_URL and BASE_URL to use conditional assignment to allow sitemap.xml to use proper domain name.

Right now https://helm.sh/sitemap.xml looks like this

This XML file does not appear to have any style information associated with it. The document tree is shown below.
<urlset>
<url>
<loc>http://localhost:3000/blog</loc>
<changefreq>weekly</changefreq>
<priority>0.5</priority>
</url>
<url>
<loc>
http://localhost:3000/blog/2019/10/22/helm-2150-released
</loc>
<changefreq>weekly</changefreq>
<priority>0.5</priority>
</url>
…
<url>
<loc>http://localhost:3000/</loc>
<changefreq>weekly</changefreq>
<priority>0.5</priority>
</url>
</urlset>

This ☝️ prevents search engines from indexing the site.

[TerryHowe]

The preview with your change still has localhost https://deploy-preview-1954--helm-merge.netlify.app/sitemap.xml

[Andygol]

I made similar changes in my own branch and deployed them on netlify and got this https://andygol-helm.netlify.app/sitemap.xml

[TerryHowe]

/lgtm

[TerryHowe]

Looks better now, thanks!!

[Andygol]

I hope this PR solved Issue #1913

[scottrigby]

Thank you!