Don't poll the github API for development builds

[ajessup]

The "Get SPIRE" section of the spiffe.io website calls the github releases API to build the releases download page. Unfortunately as this API has a strict limit for unauthenticated callers, and it's triggered on each site rebuild, it's possible to exceed the limit. This in turn breaks site building.

The suggested fix is to ensure this API is not called when in "development mode". Ideally this would be accomplished by an optional CLI flag provided to hugo that when specified would cause the site to skip calling the GH releases API.