Respect context in file cache Get() and Head()

[glin]

For https://github.com/rstudio/package-manager/issues/16862

Fix an issue with the file cache's Get() and Head() not respecting context, e.g. not properly timing out given a context with timeout.

[glin]

Note: there's still an issue with file cache Get() can hang forever due to a race condition, but that is looking trickier to fix so I may do that separately.

Another issue is why the Cache.Get() calls hang in the first place when the first node should've retrieved the cache result already. Possibly a race condition where the cache job finishes after AddressedPush(), but before PollAddress(), so the node never gets notified of the cache job completing, and just hangs:

platform-lib/pkg/rscache/file.go

Lines 275 to 290 in fd791f1

	// Push a job into the queue. AddressedPush is a no-op if the queue
	// already contains an item with the same address.
	err = o.queue.AddressedPush(ctx, resolver.Priority, resolver.GroupId, address, resolver.Work)
	if o.duplicateMatcher.IsDuplicate(err) {
	// Do nothing since; someone else has already inserted the work we need.
	slog.Debug(fmt.Sprintf("FileCache: duplicate address push for '%s'", address))
	} else if err != nil {
	value = &CacheReturn{
	Err: err,
	CacheKey: address,
	}
	return
	}
	// Find out when the job in the queue is done
	errCh := o.queue.PollAddress(ctx, address)

[CDRayn]

Looks good to me. Approved.