cmd/devp2p/ethtest: fix swapped args in private-key error message

[ozpool]

While reading the ethtest sender-loading path I noticed the error for a failed key decode passes its arguments in the wrong order:

pk, err := crypto.HexToECDSA(common.Bytes2Hex(acc.Key))
if err != nil {
    return nil, fmt.Errorf("unable to read private key for %s: %v", err, addr)
}

The format string is for %s: %v (address, then reason), but the arguments are (err, addr). So the message prints the decode error where the account address should be and the address where the reason should be, e.g.:

unable to read private key for invalid hex character 'x' in private key: 0xAbC...

Swapping to (addr, err) makes it read as intended:

unable to read private key for 0xAbC...: invalid hex character 'x' in private key

go build ./cmd/devp2p/... and go vet ./cmd/devp2p/internal/ethtest/ pass.