wgengine/magicsock: only used cached results for GetLastNetcheckReport

When querying for an exit node suggestion, occasionally it triggers a
new report concurrently with an existing report in progress. Generally,
there should always be a recent report or one in progress, so it is
redundant to start one there, and it causes concurrency issues.

Fixes #12643

Change-Id: I66ab9003972f673e5d4416f40eccd7c6676272a5
Signed-off-by: Adrian Dewhurst <adrian@tailscale.com>
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f