download: fix ignored OCI downloader settings (#9113)

Running the unused linter with field-writes-are-uses set to false, which
is not enabled in .golangci.yaml, flags five fields in this package that
are only ever written. Two of them are real bugs.

OCIDownloader.WithSizeLimitBytes and WithBundlePersistence stored their
arguments in fields download() never read, so size_limit_bytes went
unenforced for OCI bundle sources and the reader's guard on persisted
delta bundles could never fire. Wire both through as Downloader does.

The rest are dead: the trigger channel on both downloaders, unused since
Trigger() calls oneShot directly, and Downloader.stopped, dead since
Stop() moved to sync.Once. TestStopWithMultipleCalls asserted on the
latter, a check that could not fail; it now asserts the Stop calls
return, the deadlock it guards.

Signed-off-by: Sebastian Spaink <sebastianspaink@gmail.com>