gix: switch from http-curl/openssl to http-reqwest/rustls

[syphar]

the build on our server breaks because the ubuntu version we run (18.04) has a quite old openssl version (1.1.1), and libcurl needs 3.0.

  cargo:warning=curl/lib/vtls/openssl.c:101:6: error: #error "OpenSSL 3.0.0 or later required"
  cargo:warning= #    error "OpenSSL 3.0.0 or later required"
  cargo:warning=      ^~~~~

I think temporarily switching to reqwest / rustls is better, until we run on the new infra.