Resolves#430685
This was the only user of `fetchurl` which was using `-r`, which is incompatible with `--continue-at -` from 4d7a4fbd9f.
Rather than engineer a way to pass range options to `fetchurl`, let's just make a custom builder for this derivation.
It actually makes the whole thing more clear.
(cherry picked from commit 252b0b5c33)
2b26158aaf