component/curl: use slapos' ca-certificates

otherwise curl uses system ca-certificates if the system package is
installed and otherwise is not able to verify server certificates,
usually being observed in slapos as git refusing to clone with error:
SSL certificate problem: unable to get local issuer certificate

component/curl/buildout.cfg

@@ -10,6 +10,7 @@ extends =
   ../zstd/buildout.cfg
   ../zlib/buildout.cfg
   ../nghttp2/buildout.cfg
+  ../ca-certificates/buildout.cfg
 parts =
   curl
 
@@ -38,6 +39,7 @@ configure-options =
   --disable-alt-svc
   --with-zlib=${zlib:location}
   --with-ssl=${openssl:location}
+  --with-ca-path=${openssl:location}/etc/ssl/certs
   --without-gnutls
   --without-polarssl
   --without-mbedtls