Fix `http2 = false` having no effect. Fixes #2971.
Fix `http2 = false` having no effect. Fixes #2971.

Setting `http2 = false` in nix config (e.g. /etc/nix/nix.conf)
had no effect, and `nix-env -vvvvv -i hello` still downloaded .nar
packages using HTTP/2.

In `src/libstore/download.cc`, the `CURL_HTTP_VERSION_2TLS` option was
being explicitly set when `downloadSettings.enableHttp2` was `true`,
but, `CURL_HTTP_VERSION_1_1` option was not being explicitly set when
`downloadSettings.enableHttp2` was `false`.

This may be because `https://curl.haxx.se/libcurl/c/libcurl-env.html` states:
"You have to set this option if you want to use libcurl's HTTP/2 support."
but, also, in the changelog, states:
"DEFAULT
Since curl 7.62.0: CURL_HTTP_VERSION_2TLS
Before that: CURL_HTTP_VERSION_1_1"

So, the default setting for `libcurl` is HTTP/2 for version >= 7.62.0.

In this commit, option `CURLOPT_HTTP_VERSION` is explicitly set to
`CURL_HTTP_VERSION_1_1` when `downloadSettings.enableHttp2` nix config
setting is `false`.

This can be tested by running `nix-env -vvvvv -i hello | grep HTTP`

pikajude/nix,ehmry/nix,dtzWill/nix,rycee/nix,ehmry/nix,rycee/nix,ehmry/nix,rycee/nix,NixOS/nix,NixOS/nix,rycee/nix,rycee/nix,pikajude/nix,ehmry/nix,NixOS/nix,dtzWill/nix,NixOS/nix,dtzWill/nix,NixOS/nix,ehmry/nix,pikajude/nix,dtzWill/nix,ehmry/nix,dtzWill/nix