File containing environment variables to substitute when copying the configuration out of Nix store to the services.mautrix-meta.dataDir.
Can be used for storing the secrets without making them available in the Nix store.
For example, you can set services.mautrix-meta.settings.appservice.as_token = "$MAUTRIX_META_APPSERVICE_AS_TOKEN" and then specify MAUTRIX_META_APPSERVICE_AS_TOKEN="{token}" in the environment file. This value will get substituted into the configuration file as as token.