nixos/frigate: set LIBAVFORMAT_VERSION_MAJOR (#492023)

      environment = {

        CONFIG_FILE = "/run/frigate/frigate.yml";

        HOME = "/var/lib/frigate";

        # Extract libavformat version in the same way Docker scripts in frigate directory do. This

        # environment variable changes the flags given to `ffmpeg` improving compatibility.

        LIBAVFORMAT_VERSION_MAJOR = lib.strings.trim (

          builtins.readFile (

            pkgs.runCommandLocal "libavformat-major-version" { } ''

              ${cfg.settings.ffmpeg.path}/bin/ffmpeg -version | grep -Po "libavformat\W+\K\d+" > $out

            ''

          )

        );

        PYTHONPATH = cfg.package.pythonPath;

      }

      // optionalAttrs (cfg.vaapiDriver != null) {