build: enable unconvert linter (#15456)

* build: enable unconvert linter

 - fixes #15453
 - update code base for failing cases

* cmd/puppeth: replace syscall.Stdin with os.Stdin.Fd() for unconvert linter

common/hexutil/hexutil.go

@@ -53,9 +53,7 @@ var (
 
 type decError struct{ msg string }
 
-func (err decError) Error() string {
-	return string(err.msg)
-}
+func (err decError) Error() string { return err.msg }
 
 // Decode decodes a hex string with 0x prefix.
 func Decode(input string) ([]byte, error) {

common/hexutil/json.go

@@ -223,7 +223,7 @@ func (b *Uint64) UnmarshalText(input []byte) error {
 			return ErrSyntax
 		}
 		dec *= 16
-		dec += uint64(nib)
+		dec += nib
 	}
 	*b = Uint64(dec)
 	return nil