Explicitly specify the path to the easy-rsa vars file

[smokris]

Pull Request (PR) description

When puppet-openvpn runs ./easyrsa gen-crl, it fails with:

Found: /opt/local/etc/easyrsa/vars
Found: /opt/local/etc/openvpn/someName/easy-rsa/vars

Easy-RSA error:

Conflicting 'vars' files found.

Priority should be given to your PKI vars file:
* /opt/local/etc/openvpn/someName/easy-rsa/pki/vars

EasyRSA Version Information
Version:     3.1.4
Generated:   Wed May 24 07:02:30 CDT 2023
SSL Lib:     OpenSSL 3.0.8 7 Feb 2023 (Library: OpenSSL 3.0.8 7 Feb 2023)
Git Commit:  a7284a1857dcd157d5f606561c86cb79c9c5091f
Source Repo: https://github.com/OpenVPN/easy-rsa
Host: 3.1.4 | nix | SunOS | /bin/bash

This PR sets the EASYRSA_VARS_FILE environment variable, so that easyrsa knows which vars file to use.

This Pull Request (PR) fixes the following issues

n/a