Fixed sys usage (#916)

Just as a heads up, I haven't really tested this since a lot of it is
stuff I don't use or know how to set up without some reading up. I have
tested the nu_conda_2 change since I have a python project that I use
that for, and I could look into testing more of it if needed.

I've tried finding (naively using `/sys\W/`) all the usage of the old
plain `sys` calls and replacing them with alternates as appropriate,
which mostly has been to swap a `(sys).host.name` call into a
`$nu.os-info.name` one, since it'll be tad faster and more consistent
across platforms with naming (especially as the value comes from the
[rust stdlib](https://doc.rust-lang.org/std/env/consts/constant.OS.html)
and is very predictable).

Fixes #897

modules/virtual_environments/auto-venv/venvs/python-venv.nu

@@ -21,7 +21,7 @@ export def --env auto-venv-on-enter [
     let bin            = ([$virtual_env, "bin"] | path join)
     let virtual_prompt = ""
 
-    let is_windows = ((sys).host.name | str downcase) == 'windows'
+    let is_windows = $nu.os-info.name == 'windows'
     let path_name = if $is_windows {
         if (has-env 'Path') {
             'Path'