while refactoring the curl wrapper we inadvertently broken the immutable
flake protocol, because the immutable flake protocol accumulates headers
across the entire redirect chain instead of using only the headers given
in the final response of the chain. this is a problem because Some Known
Providers Of Flake Infrastructure set rel=immutable link headers only in
the penultimate entry of the redirect chain, and curl does not regard it
as worth returning to us via its response header enumeration mechanisms.
fixes https://git.lix.systems/lix-project/lix/issues/358
Change-Id: I645c3932b465cde848bd6a3565925a1e3cbcdda0
dd4a2c1759