Dynamic var to customize env at task creation time

It would be nice to shadow the global env to better separate multiple apps running in the same boot pipeline. This shadowing is only needed at task creation time, not run time.

As discussed in Slack a dynamic var *env* (in boot.pod or boot.core) might be the solution.

;; binding *env* when creating the pipeline
(binding [*env* (get-env)]
  (my-task))

(deftask my-task []
  (fn [next-task]
    ;; bind env outside fileset-receiving fn
    (let [env *env*]
      (fn [fileset]
        (pod/make-pod env ,,,))))

/cc @micha @arichiardi