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
6 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
passed SlapOS.Eggs.UnitTest-Master.Python2

00:16:19

passed SlapOS.Eggs.UnitTest-Master.Python3

00:13:01

passed SlapOS.SlapRunner.ResilienceTest-Master

00:24:49

failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5

02:20:14

failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python2

04:34:37

failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python3

06:08:23