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_unix.go

@@ -0,0 +1,14 @@
+//go:build linux || darwin || !windows
+// +build linux darwin !windows
+
+package in_toto
+
+import "golang.org/x/sys/unix"
+
+func isWritable(path string) error {
+	err := unix.Access(path, unix.W_OK)
+	if err != nil {
+		return err
+	}
+	return nil
+}