When using a custom CA certificate, add it to the system's list of CA certificates

[makkes]

Currently, when specifying a custom CA file in a GitRepository's secret ref or HelmRepository's secret ref, then any system certificates already present in the container running source-controller are discarded. This is a problem when a mix of sources with custom and public CAs is used, e.g. in the case where a Helm in a repository hosted on GitHub (public CA) chart depends on another charts hosted in a private repository (custom CA) as discussed here.

Source-controller, when configured with a custom CA file should add the custom CA file on top of any existing CA files it already knows.