cmd/cue/cmd: fix windows test breakage

Change-Id: Ic338fcdb5b06fd95f9d57e17b5fe04bd13989dcc
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/6144
Reviewed-by: CUE cueckoo <cueckoo@gmail.com>
Reviewed-by: Paul Jolly <paul@myitcv.org.uk>
diff --git a/internal/cuetxtar/txtar.go b/internal/cuetxtar/txtar.go
index 0164605..f2b6804 100644
--- a/internal/cuetxtar/txtar.go
+++ b/internal/cuetxtar/txtar.go
@@ -117,7 +117,7 @@
 	if err != nil {
 		return filepath.Base(filename)
 	}
-	return rel
+	return filepath.ToSlash(rel)
 }
 
 // WriteErrors writes strings and