When an explicit backend is configured with a configuration which has
not yet been initialized, running `terraform init` performs a state
migration to fetch the remotely stored state in order to operate on it.
Like the previous bug introduced by the recent provider diagnostics
change, this code path was not correctly configured to enable init mode
for the backend, which resulted in a fatal error during init when the
cache dir is deleted.
Setting the `Init` backend option allows this code path to continue
without error when first initializing the backend for state migration.
The new e2e test fails without this change.
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