Fix input/output file comparison when signing

Stat doesn't return ErrExist error, so this line is always false. We
also don't need any stats for the output file, so we can just try to
open the file, and continue the comparison with the input file if
there is no error opening it.

keys.go

@@ -66,11 +66,7 @@ func SignFile(state *config.State, kh *backend.KeyHierarchy, ev hierarchy.Hierar
 
 	// Check if the files are identical
 	if file != output {
-		if _, err := state.Fs.Stat(output); errors.Is(err, os.ErrExist) {
-			outputFile, err := state.Fs.Open(output)
-			if err != nil {
-				return err
-			}
+		if outputFile, err := state.Fs.Open(output); err == nil {
 			defer outputFile.Close()
 			outputBinary, err := authenticode.Parse(outputFile)
 			if err != nil {