Ignore signatures when using oc-mirror

[bfournie]

Nightly builds don't include signatures so ignore when using oc-mirror.

[openshift-ci]

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[openshift-ci]

@bfournie: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
ci/prow/e2e-agent-4control-ipv4	a7af017	link	false	/test e2e-agent-4control-ipv4
ci/prow/e2e-agent-sno-ipv6	a7af017	link	false	/test e2e-agent-sno-ipv6
ci/prow/images	a7af017	link	true	/test images
ci/prow/e2e-metal-ipi-ovn-dualstack	a7af017	link	false	/test e2e-metal-ipi-ovn-dualstack
ci/prow/e2e-metal-ipi-bm-bond	a7af017	link	false	/test e2e-metal-ipi-bm-bond
ci/prow/e2e-metal-ipi-virtualmedia	a7af017	link	false	/test e2e-metal-ipi-virtualmedia
ci/prow/e2e-agent-bad-dns	a7af017	link	false	/test e2e-agent-bad-dns
ci/prow/e2e-metal-ovn-arbiter	a7af017	link	false	/test e2e-metal-ovn-arbiter
ci/prow/e2e-agent-5control-ipv4	a7af017	link	false	/test e2e-agent-5control-ipv4
ci/prow/e2e-metal-ipi-serial-ovn-ipv6	a7af017	link	false	/test e2e-metal-ipi-serial-ovn-ipv6
ci/prow/e2e-metal-ipi-bm	a7af017	link	true	/test e2e-metal-ipi-bm
ci/prow/e2e-agent-ha-dualstack	a7af017	link	false	/test e2e-agent-ha-dualstack
ci/prow/e2e-metal-ipi-ovn-ipv6	a7af017	link	true	/test e2e-metal-ipi-ovn-ipv6
ci/prow/e2e-agent-compact-ipv4	a7af017	link	true	/test e2e-agent-compact-ipv4
ci/prow/e2e-agent-compact-ipv4-iso-no-registry	a7af017	link	false	/test e2e-agent-compact-ipv4-iso-no-registry
ci/prow/e2e-metal-ipi-serial-ipv4-2of2	a7af017	link	true	/test e2e-metal-ipi-serial-ipv4-2of2
ci/prow/e2e-metal-ipi-serial-ipv4-1of2	a7af017	link	true	/test e2e-metal-ipi-serial-ipv4-1of2

Full PR test history. Your PR dashboard.

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[cybertron]

/lgtm

CI failures look related to ansible galaxy, not the patch.