Copy installed plugins does not take `PLUGIN_DIR` ENV into account

[casabre]

Describe the bug

I had to set the PLUGIN_DIR ENV in order to avoid the issue #506. This is working fine so far but the copying stage fails with cp: cannot stat '/usr/share/jenkins/ref/plugins/*': No such file or directory

Version of Helm and Kubernetes

- Helm: unknown
- Kubernetes: v1.28.10+a2c84a5

Chart version

5.3.6

What happened?

I wanted to install just the CasC plugin through the initContainer. The downloading process worked fine but the copying process broke because it was looking in the wrong folder.

Having an empty list for `installPlugins` works fine because it is skipping the whole procedure

What you expected to happen?

Copying the downloaded and installed plugins from the correct PLUGIN_DIR folder

How to reproduce it

controller:
  initContainerEnv:
    - name: CACHE_DIR
      value: "/tmp/cache"
    - name: PLUGIN_DIR
      value: "/tmp/plugins"
  installPlugins:
    - configuration-as-code:1810.v9b_c30a_249a_4c

Anything else we need to know?

Had to strip some proxy and security settings because their are corporate internally. But I believe this shouldn't make a difference.