fix: cache the update check timestamp when the version lookup fails

[arose26]

Fixes #3840

get_latest_version() returned early when the PyPI lookup raised, before writing
the last-check entry to the self-check cache. A failed check was therefore never
cached, so every subsequent command retried the request and waited out the
connection timeout again — making pdm appear to hang for users who are offline or
behind a restrictive network.

This records the check time whether or not the lookup succeeded, so a failure
backs off for the same expiry window as a success. latest-version is still only
written on success, so the cached-hit path now reads it with .get().

Added a regression test asserting that a second call within the expiry window does
not re-hit the network, plus a news fragment.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.36%. Comparing base (5a6ea88) to head (a8d5940).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3846   +/-   ##
=======================================
  Coverage   88.36%   88.36%           
=======================================
  Files         121      121           
  Lines       13249    13249           
  Branches     2251     2251           
=======================================
  Hits        11708    11708           
  Misses        969      969           
  Partials      572      572           

Flag	Coverage Δ
unittests	88.25% <ø> (ø)

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