don’t call whoami so many times

This patch just uses the `USER` environment variable instead of shelling out to whoami all the time.

* We're getting sporadic out of memory issues on our server. While I don't think calling `whoami` a ton of times is causing this issue, it is aggravating it.
* This may fix `whoami` support on windows too.
* The performance increases I got here were less than the margin for error on timing.

Thanks

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If you would prefer not to cache this value in `@user`, then I can do that, too.