retryable_download: preserve partial download on network errors

`RetryableDownload#fetch` cleared the download cache before every retry,
which removes the `.incomplete` file that `CurlDownloadStrategy` uses
for resumable downloads. As a result `--continue-at -` never kicked in:
each retry restarted from zero, making installs on unstable networks
effectively impossible once a bottle was large enough to never finish
within one uninterrupted session.

Only clear the cache when the fully-downloaded file is known-bad
(`ChecksumMismatchError` or `Resource::BottleManifest::Error`). For
network-level `DownloadError`, keep the partial `.incomplete` file so
the next attempt can resume via `--continue-at`. If that partial was
itself corrupted mid-flight, the post-download checksum check will
catch it on a later iteration and clear the cache via the existing
path, so the loop is self-healing.

Closes #21518.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>