build: report docker feature detection failures

[crazy-max]

fix partially #3266

Return Docker feature detection errors instead of reporting unsupported manifest lists, and cache only successful results per Docker context so failed probes can be retried. Probe the builder daemon for the docker driver and the load destination for other drivers, while skipping daemon detection for tarball exports.

The driver-level ListWorkers feature cache has a similar error-handling issue and can be addressed in a follow-up:

buildx/driver/docker/driver.go

Lines 83 to 103 in ddf35fc

	d.features.once.Do(func() {
	var useContainerdSnapshotter bool
	if c, err := d.Client(ctx); err == nil {
	workers, _ := c.ListWorkers(ctx)
	for _, w := range workers {
	if _, ok := w.Labels["org.mobyproject.buildkit.worker.snapshotter"]; ok {
	useContainerdSnapshotter = true
	}
	}
	c.Close()
	}
	d.features.list = map[driver.Feature]bool{
	driver.OCIExporter: useContainerdSnapshotter,
	driver.DockerExporter: useContainerdSnapshotter,
	driver.CacheExport: useContainerdSnapshotter,
	driver.MultiPlatform: useContainerdSnapshotter,
	driver.DirectPush: useContainerdSnapshotter,
	driver.PreferImageDigest: useContainerdSnapshotter,
	driver.DefaultLoad: true,
	}
	})

Its Features interface cannot return errors, so simply allowing failed probes to retry could change capability results midway through a build. An early failure could disable default provenance, while a later successful probe allows the build to proceed without it. Fixing that safely requires propagating detection errors through the driver interface and its callers, which is broader than this targeted fix.

[tonistiigi]

Potential follow ups from agents:

• Docker driver feature detection (line 82) still caches a failed ListWorkers probe as unsupported. #4085 should address this before #3266 is considered fully resolved.
• The tests do not run concurrent feature lookups despite the commit’s coverage claim. The error fixtures in opt_test.go (line 88) and client_test.go (line 122) also contain invalid JSON.