Updates buildkit to current master (github.com/moby/buildkit): moby/buildkit@be6da00...7a4a2a2

Fixes build failures due to gRPC bumps;
18:54:28 vendor/github.com/moby/buildkit/session/filesync/filesync.go:185:8:warning: NewContext not declared by package metadata (gosimple)
18:54:28 vendor/github.com/moby/buildkit/session/filesync/filesync.go:69:13:warning: FromContext not declared by package metadata (gosimple)
18:54:28 vendor/github.com/moby/buildkit/session/filesync/filesync.go:185:8:warning: NewContext not declared by package metadata (interfacer)
18:54:28 vendor/github.com/moby/buildkit/session/filesync/filesync.go:69:13:warning: FromContext not declared by package metadata (interfacer)
18:54:28 vendor/github.com/moby/buildkit/session/filesync/filesync.go:69:13:warning: FromContext not declared by package metadata (unconvert)
18:54:28 vendor/github.com/moby/buildkit/session/filesync/filesync.go:185:8:warning: NewContext not declared by package metadata (unconvert)

Update vendored dependencies to match BuildKit:

- add github.com/grpc-ecosystem/grpc-opentracing dependency
- add github.com/opentracing/opentracing-go dependency
- downgrade github.com/pkg/errors to a tagged release: pkg/errors@v0.8.0...839d9e9
- github.com/containerd/continuity containerd/continuity@d8fb858...3e8f2ea
  - containerd/continuity#110 Add fstest.CreateSocket
- github.com/containerd/fifo containerd/fifo@fbfb6a1...3d5202a
  - Add apache license to files
- github.com/containerd/go-runc containerd/go-runc@4f6e87a...f271fa2
  - containerd/go-runc#37 runc: be able to get the full ps data (ps -f table)
  - containerd/go-runc#40 Add ConsoleSocket to RestoreOpts
- github.com/containerd/console containerd/console@2748ece...cb7008a
  - containerd/console#21 Add OpenBSD support
- github.com/syndtr/gocapability syndtr/gocapability@2c00dae...db04d3c
  - syndtr/gocapability#11 Add support for ambient capabilities
  - syndtr/gocapability#13 Fix issue moby#12: break too early
- golang.org/x/net golang/net@5561cd9...0ed95ab
- golang.org/x/text golang/text@f72d839...19e5161
- golang.org/x/time golang/time@a4bde12...f51c127
- github.com/tonistiigi/fsutil tonistiigi/fsutil@dea3a0d...93a0fd1
  - tonistiigi/fsutil#16 Don not hang on diskwriter errors
  - tonistiigi/fsutil#17 fix fd leak on send
  - tonistiigi/fsutil#18 avoid possible receiver panic/deadlock on sender error
  - tonistiigi/fsutil#20 receive: read stream to EOF on close
  - tonistiigi/fsutil#21 avoid not-exist error on walker
  - tonistiigi/fsutil#27 Generalize chtimes() implementation for non-linux platforms
  - tonistiigi/fsutil#28 walker: handle parents and subdirs in includepatterns

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>