Add support for custom .env file path

[TinfoilSubmarine]

Hi! I'm working on packaging this for Void, and in writing the runit service, I came across an annoyance: since the .env file can't be in the current working directory, it won't find it and thus has to read from the environment variables directly. This can be done with something like:

#!/bin/sh
exec chpst -u _vaultwarden:_vaultwarden \
        env $(grep -v '^#' ${ENV_FILE:-/etc/vaultwarden.env} | xargs -d '\n') vaultwarden

But that is less than ideal IMO. This patch is a simple way to pass a single custom location where the env/config can be found.

[jjlin]

There should also be some docs towards the top of https://github.com/dani-garcia/vaultwarden/blob/main/.env.template so people know this env var is available

[TinfoilSubmarine]

I should mention that another solution to this is to cd to the directory containing the .env file (or a symlink to it) prior to executing vaultwarden. So my service file would look like:

ln -sf ${ENV_FILE:-/etc/vaultwarden.env} /var/lib/vaultwarden/.env
cd /var/lib/vaultwarden
exec chpst -u _vaultwarden:_vaultwarden vaultwarden

This works for my use case, but I'll leave this PR here in case it's seen as a benefit.