Fix -only32bit flag in `Build-NWJS.ps1`

In #367 I forced building 64-bit + WinXP (32bit) which is not a configuration the script is supposed to support (I forgot about the -only32bit flag). So the script (although it works in the build pipeline) is currently left in a confusing state. I should probably change the flag to -no32bit, and make sure that if it's evoked without the flag, all packages are built.