nix: modules/spindle: Fix nix option to match spindle

There was a change the environment variables in spindle that were not
reflected in this nix module, specifically STEP_TIMEOUT was renamed to
WORKFLOW_TIMEOUT, which means that there was no way to set the timeout
in the nix module.

With the option name being updated, this is a breaking change that will
require you to update your nix config if you are using the spindle module.

Signed-off-by: Samuel Shuert <[e-mail]>

nix/modules/spindle.nix

···
             description = "Nixery instance to use";
           };
 
-          stepTimeout = mkOption {
+          workflowTimeout = mkOption {
             type = types.str;
             default = "5m";
             description = "Timeout for each step of a pipeline";
···
             "SPINDLE_SERVER_DEV=${lib.boolToString cfg.server.dev}"
             "SPINDLE_SERVER_OWNER=${cfg.server.owner}"
             "SPINDLE_PIPELINES_NIXERY=${cfg.pipelines.nixery}"
-            "SPINDLE_PIPELINES_STEP_TIMEOUT=${cfg.pipelines.stepTimeout}"
+            "SPINDLE_PIPELINES_WORKFLOW_TIMEOUT=${cfg.pipelines.workflowTimeout}"
           ];
           ExecStart = "${self.packages.${pkgs.system}.spindle}/bin/spindle";
           Restart = "always";