Users frequently mis-spell names of fields or misplace fields in the install-config. Loading the install-config using UnmarshalStrict() would allow us to catch these errors and fail immediately, enabling users to correct their input before they end up opening a support case to figure out why something they thought they specified is not taking effect.

This was previously enabled in https://github.com/openshift/installer/pull/5307 but it had to be reverted to a warning (https://bugzilla.redhat.com/show_bug.cgi?id=2098299) because Hive may generate an install-config for an older version of the installer using the vendored MachineConfig from a newer version of the installer with additional fields added.

We can keep the current behaviour for Hive by checking the OPENSHIFT_INSTALL_INVOKER environment variable. Limiting it to a warning whenever that env var is non-empty would ensure that neither Hive nor assisted-service would see any change in behaviour that could break them. For everyone else, we should return an error when strict unmarshalling fails.