vendor: update buildkit to v0.8

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

vendor/github.com/moby/sys/mount/mount_errors.go

@@ -0,0 +1,46 @@
+// +build !windows
+
+package mount
+
+import "strconv"
+
+// mountError records an error from mount or unmount operation
+type mountError struct {
+	op             string
+	source, target string
+	flags          uintptr
+	data           string
+	err            error
+}
+
+func (e *mountError) Error() string {
+	out := e.op + " "
+
+	if e.source != "" {
+		out += e.source + ":" + e.target
+	} else {
+		out += e.target
+	}
+
+	if e.flags != uintptr(0) {
+		out += ", flags: 0x" + strconv.FormatUint(uint64(e.flags), 16)
+	}
+	if e.data != "" {
+		out += ", data: " + e.data
+	}
+
+	out += ": " + e.err.Error()
+	return out
+}
+
+// Cause returns the underlying cause of the error.
+// This is a convention used in github.com/pkg/errors
+func (e *mountError) Cause() error {
+	return e.err
+}
+
+// Unwrap returns the underlying error.
+// This is a convention used in golang 1.13+
+func (e *mountError) Unwrap() error {
+	return e.err
+}