vaultwarden: Add VW_VERSION env to fix version detection
At build time, vaultwarden looks at the VW_VERSION env variable
to embed it's own version string. When this is unset, it still compiles,
but responds with "null" as version info under `/api/config`, which leads
to issues like https://github.com/dani-garcia/vaultwarden/discussions/3996
Under Nix, this variable is unset, so this commit properly forces it
