Installing a plugin always causes jar-dependencies to try to access maven

When online on Ubuntu or CentOS, installing a plugin causes jar-dependencies to try to access the local ~/.m2 configuration, which may not exist, causing an error:

```
osboxes@osboxes:~$ sudo -u logstash /usr/share/logstash/bin/logstash-plugin install logstash-filter-truncate
Validating logstash-filter-truncate
Installing logstash-filter-truncate
Error Bundler::InstallError, retrying 1/10
An error occurred while installing logstash-core (5.5.2), and Bundler cannot continue.
Make sure that `gem install logstash-core -v '5.5.2'` succeeds before bundling.
```

Since the plugins that use jars now ship with them inside the gem, we can instruct jar-dependencies to leave the .m2 folder alone using JARS_SKIP=true.