Use java.io.tmpdir instead of user.home for storing extracted library

[tarioch]

Currently the LibraryLoader uses "user.home" as the fallback, it would be nicer if it would use "java.io.tmpdir" instead.

[kernle32dll]

Well, you can do this pretty easily yourself:

String tempDirectory;

try {
  tempDirectory = Files.createTempDirectory("J2V8_temp").toString();
} catch (final IOException e) {
  // Temp dir creation failed - use J2V8 default
  tempDirectory = null;
}

V8.createV8Runtime(null, tempDirectory)

This creates a temporary folder "J2V8_temp" inside the systems tmp folder. We recently fell over this, because on Alpine (#252) user.home defaults to "/" - which fails to write to (at least in our Docker context).

[tarioch]

In my case I'm actually using another library which in turn uses j2v8, but even though I can probably workaround it by manually initializing j2v8 first and then only use the other library.
But I think it still makes sense to change the default to be the tmpdir.

[matiwinnetou]

I agree that default makes sense to tempDir. This is very easy PR you could contribute this @tarioch