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.

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

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

/cc @micha @arichiardi

Start by reading the environment and task-creation code in boot.pod or boot.core, then trace get-env and pod/make-pod in the example. Done means a dynamically bound *env* can separate pipelines during task creation without changing the environment at run time; the issue does not name a test to run.

Written by the indexing model from the issue text.