openssl, curl, git: Respect $NIX_SSL_CERT_FILE

$NIX_SSL_CERT_FILE overrides $SSL_CERT_FILE, which in turn overrides
the default CA path (/etc/ssl/certs/ca-certificates.crt). This allows
Nix to set a CA path without interfering with other packages (such as
Homebrew).

See https://github.com/NixOS/nix/issues/921.