use crates.io as source for substrate git deps

[clangenb]

I don't really see a reason not to do this except for lack of resources. :)

[clangenb]

Also consider that we might want to introduce the subxt dependency and the xcm test again at this point, see #230.

[brenzi]

can you clarify plz? I don't understand what this issue is about

[clangenb]

Sorry, was a hasty PR description, and it is even wrong. I wanted to say switch to crates.io as source for substrate deps.

[brenzi]

done