When manually adding the source tarball to the store with
`nix-prefetch-url`, it can be GC'd after the derivation is built. Then
if any library version updates and the derivation wants to rebuild, the
same tarball needs to be redownloaded and re-prefetched. (Even if
downloading it automatically, this can be annoying on a slower network
connection, since it's nearly 4GB).
This advice is mostly helpful on NixOS and it'd be nice to offer
something to non-NixOS nix users as well, but I don't know what it would
be.
