snap: Set "extended" tag based on build_url on Launchpad
snap: Set "extended" tag based on build_url on Launchpad

Read build_url from $SNAPCRAFT_IMAGE_INFO to determine whether
to add the "extended" build tag or not.  This allows us to do away
with the extended-snap-master and extended-snap-stable branches.

anthonyfok/hugo,mdhender/hugo,mdhender/hugo,fedelibre/hugo,mdhender/hugo,fedelibre/hugo,gohugoio/hugo,gohugoio/hugo,anthonyfok/hugo,gohugoio/hugo,mdhender/hugo,fedelibre/hugo,fedelibre/hugo,anthonyfok/hugo