cue: fix Windows build breakage

Take canonical representation of paths.

Change-Id: I1d3c48b0bc06f2e4d58db72b7f73931b303881ab
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/2943
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>
diff --git a/cue/marshal.go b/cue/marshal.go
index 843f018..5a69ea5 100644
--- a/cue/marshal.go
+++ b/cue/marshal.go
@@ -166,6 +166,7 @@
 			}
 			if dir != "" {
 				filename, _ = filepath.Rel(dir, filename)
+				filename = filepath.ToSlash(filename)
 			}
 		}