Need to set HELM_PLUGIN for helm plugin support

[jessesuen]

Even if a user customizes their repo server through a custom Dockerfile, they will not benefit from installing helm plugins, because we create a temporary, throwaway HELM_HOME whenever we run helm template. In order to support plugins, we should set HELM_PLUGIN to /home/argocd/.helm/plugins so that HELM_HOME can be a temp directory, and HELM_PLUGIN will be the customized location.

One approach might be to just change the Dockerfile to have the HELM_PLUGIN set to home/argocd/.helm/plugins and we inherit these environment variables when running helm template.

Another approach is to just set the HELM_PLUGIN environment variable for all fork/execs.

[jessesuen]

because we create a temporary, throwaway HELM_HOME whenever we run helm template

In case anyone is wondering why we do this, this is because Argo CD helm repository list dynamic, and the full list is supplied on-request for each GetManifests call. This allows the repo server to run without K8s service account & token. So if the repo-server is compromised (which will be more possible with custom templating), at least it cannot talk to the K8s API server.

Here is helm docs on it's env vars:
https://github.com/helm/helm/blob/master/docs/plugins.md#environment-variables

[alexec]

#1105

[jessesuen]

Should be now possible to do this with fix in #1306. Requires a custom repo server which sets HELM_PLUGIN environment variable.