Name conflict between Jlink and native build

[tomas-langer]

When I build Jlink and then native image, such as:

helidon build --mode JLINK
helidon build --mode NATIVE

The second command fails, because native image refuses to create a file with the same name as an existing directory.

I think we should rename the directory.

Then our options are:
PLAIN: target/${artifactId}.jar
NATIVE: target/${artifactId}
JLINK: target/${artifactId}_jlink/*