Pass environment needed for systemd-ask-pass

[Firstyear]

systemd-ask-pass allows the pin to be provided by the systemd-credentials mechanism, but that requires the environment variable $CREDENTIALS_DIRECTORY to be set. Openvpn when launched by systemd does have this environment variable available, but it doesn't pass it through to systemd-ask-pass meaning that systemd-ask-pass is unable to load the pin from systemd-credentials.

This change passes the $CREDENTIALS_DIRECTORY environment variable to the execed systemd-ask-pass invocation so that systemd-credentials can be used for non-interactive pin entry.

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• https://community.openvpn.net/openvpn/wiki/DeveloperDocumentation
• https://community.openvpn.net/openvpn/wiki/Contributing

[Firstyear]

I won't be able to submit this to a mailing list, as I don't have a git email work flow setup. Feel free to send this on my behalf.