Fix usage of OCI target with insecure transport

[makkes]

The --insecure flag wasn't respected in oci.Repo#Has. Now, whenever that flag is set, a tls.Config with InsecureSkipVerify set to true is set injected into the DefaultTransport used to connect to the OCI registry.

Before

$ ~/dev/charts-syncer/dist/charts-syncer --insecure -c ~/dev/charts-syncer/test-config.yaml sync --latest-version-only --insecure
I0718 17:22:47.287675 3905275 sync.go:34] Using config file: "/home/max/dev/charts-syncer/test-config.yaml"
E0718 17:22:48.564152 3905275 index.go:153] unable to explore target repo to check "grafana-6.58.4" chart: failed checking remote: Get "https://localhost:8443/v2/": tls: failed to verify certificate: x509: certificate is valid for garage.garage.svc, not localhost; Get "http://localhost:8443/v2/": EOF
W0718 17:22:48.564170 3905275 index.go:120] Failed processing grafana:6.58.4 chart. The index will remain incomplete.
W0718 17:22:48.564180 3905275 sync.go:34] There were some problems loading the information of the requested charts: failed checking remote: Get "https://localhost:8443/v2/": tls: failed to verify certificate: x509: certificate is valid for garage.garage.svc, not localhost; Get "http://localhost:8443/v2/": EOF
I0718 17:22:48.564189 3905275 sync.go:49] There are no charts out of sync!

After

$ ~/dev/charts-syncer/dist/charts-syncer --insecure -c ~/dev/charts-syncer/test-config.yaml sync --latest-version-only --insecure
I0718 17:30:51.796045 3919041 sync.go:34] Using config file: "/home/max/dev/charts-syncer/test-config.yaml"
I0718 17:30:52.060203 3919041 sync.go:47] There is 1 chart out of sync!
I0718 17:30:52.060215 3919041 sync.go:55] Syncing "grafana-6.58.4" chart...

[makkes]

superseded by #190