Fix default lock file path

[Tehforsch]

This fixes a regression in which we assume that the feed update lock file is available at /var/lib/openvas/plugins/feed-update.lock, but this is a very unlikely default when the feed path is set to something other than /var/lib/openvas/plugins.

This

1. Fixes the above default to default to $FEED_PATH/feed-update.lock
2. Fixes millions of sends on a closed channel polluting the logfile, obscuring the real issue
3. Gracefully handles a missing lockfile by returning an error instead of panicking

Jira:
SC-1608
SC-1642

[github-actions]

Dependency Review

✅ No vulnerabilities or license issues or OpenSSF Scorecard issues found.

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