Fix issue with checking gem push exit code in release workflow

[jeffcharles]

The last issue with the releases happened because of the way we evaluate non-zero exit conditions is slightly wrong.

If you run this bash script, you'll see that errcode is set to 0:

if ! /usr/bin/false; then
    errcode=$?
    echo $errcode
fi

whereas with this script, you'll see that errcode is set to 1:

/usr/bin/false || errcode=$?
if [ -n $errcode ]; then
    echo $errcode
fi

I think it's because the use of the ! operator inverts the value in $?.