consider allowing variables in the upstreams key to help scriptability

[joebonrichie]

e.g. https://download.gnome.org/sources/nautilus/47/nautilus-47.2.tar.xz

major_version=$(echo 47.2 | awk -F. '{print $1}')

we can replace the url with this:

https://download.gnome.org/sources/nautilus/$major_version/nautilus-$version.tar.xz

this could potentially be sourced from the environment key