nixos/slurm: fix wrapper script variable expansion, and refactor (#470374)

  config =

    let

      wrappedSlurm = pkgs.stdenv.mkDerivation {

        name = "wrappedSlurm";

        builder = pkgs.writeText "builder.sh" ''

      wrappedSlurm = pkgs.runCommand "wrappedSlurm" { } ''

        mkdir -p $out/bin

        find  ${lib.getBin cfg.package}/bin -type f -executable | while read EXE

        do

          wrappername="$out/bin/$exename"

          cat > "$wrappername" <<EOT

        #!/bin/sh

          if [ -z "$SLURM_CONF" ]

        if [ -z "\$SLURM_CONF" ]

        then

          SLURM_CONF="${cfg.etcSlurm}/slurm.conf" "$EXE" "\$@"

        else

        mkdir -p $out/share

        ln -s ${lib.getBin cfg.package}/share/man $out/share/man

      '';

      };

    in

    lib.mkIf (cfg.enableStools || cfg.client.enable || cfg.server.enable || cfg.dbdserver.enable) {