Option to skip if Graal already made exe

[Lilianne-Blaze]

Please add an option to try to avoid building the exe if it was already created by some other plugin, like GraalVM native image plugin.

Rationale:
As it is now, if you create a simple Micronaut app and just add Launch4j with minimal/typical configuration, it will create the exe as normal with a normal build, but also during a packaging=native-image build, overwriting the one made by Graal.

I made a quick fix that adds skipIfNativeImage param that does just that, tries to detect if Graal was already executed during the current build. It could use some more testing, but it works with the projects I'm working on right now

[Lilianne-Blaze]

#374