Fix bug around spawning worker processes.

We were expecting to find the path to the `strawboss` binary implicitly from
the environment, which meant that configuration was also implicit, and required
more setup. Now the path to the binary is explicit in the StrawBoss runtime
configuration, and the path to the configuration file can also be explicitly given.

src/main/nim/strawboss.nim

@@ -41,6 +41,7 @@ Options
                 else: "strawboss.config.json"
 
   var cfg = loadStrawBossConfig(cfgFile)
+  cfg.pathToExe = paramStr(0)
   if not existsDir(cfg.artifactsRepo):
     echo "Artifacts repo (" & cfg.artifactsRepo & ") does not exist. Creating..."
     createDir(cfg.artifactsRepo)