vendor: update buildkit to master@31c870e82a48

Signed-off-by: Justin Chadwell <me@jedevc.com>

vendor/github.com/in-toto/in-toto-golang/in_toto/util_windows.go

@@ -0,0 +1,25 @@
+package in_toto
+
+import (
+	"errors"
+	"os"
+)
+
+func isWritable(path string) error {
+	// get fileInfo
+	info, err := os.Stat(path)
+	if err != nil {
+		return err
+	}
+
+	// check if path is a directory
+	if !info.IsDir() {
+		return errors.New("not a directory")
+	}
+
+	// Check if the user bit is enabled in file permission
+	if info.Mode().Perm()&(1<<(uint(7))) == 0 {
+		return errors.New("not writable")
+	}
+	return nil
+}