macOS: Reliably apply workaround for system curl 8.{3,4,5} LibreSSL backend

In commit f2596dfa0e (macOS: Work around bug in system curl 8.{3,4,5}
LibreSSL backend, 2024-07-16, v3.30.1~2^2) we tried to prefer
`secure-transport` on problematic versions of curl.  However, the
`curl_global_sslset` setting must be applied before every
`curl_global_init` call, not just the first one.  Otherwise a
second (or subsequent) download won't apply the work-around.