spire: don't scan for supervisor keys during auto-launch

[cryslith]

When launching an existing virtual cluster, don't attempt to
authenticate to the supervisor with the pre-keysystem method
of using the preseeded authorized ssh key.
Doing so won't work because that ssh key is no longer authorized
on the supervisor after the keysystem is set up.
(Previously, it sometimes appeared to work when the
keysystem-provided ssh keys were still loaded from a prior
auto-install.)

This also avoids unnecessarily pulling the host key
from the keysystem and verifying it using the fingerprint
displayed on the supervisor console; once the keysystem is
set up the supervisor can authenticate itself using the CA.

---

Checklist:

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• The first line of each commit is of the form "[component]: do something"
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• If my change includes new or modified functionality, I have tested that the changes work as expected.
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[cryslith]

bors r+
bors r-

[hyades-bors]

Canceled.

[cryslith]

bors r+

(for science)

[cryslith]

no, stop that.

bors r-

[hyades-bors]

Canceled.