Support env vars for {DESK,LAP,CLOUD}TOP devices

I recently changed my hostname for my desktop and laptop from

wpcarro.lon.corp.google.com -> zeno.lon.corp.google.com
wpcarro2                    -> seneca

If you're curious, the names Zeno and Seneca come from famous Stoic
philosophers. As you can see from this commit, my configuration depends on the
values of these hostnames.

Immediately impacted:
- .profile
- device.el

Not immediately impacted:
- configs/install
- configs/uninstall
- .ssh/config
- .zshrc*

* As a side note, I should stop supporting ZSH.

Using an .envrc file helps me DRY up some of my configuration. Ideally I should
only need to make changes to the .envrc file and then expect everything to work
as expected. Let's see how that goes.

configs/uninstall

@@ -3,14 +3,11 @@
 configs="$DOTFILES/configs"
 
 case $(hostname) in
-  # desktop
-  wpcarro.lon.corp.google.com)
+  $DESKTOP)
     (cd "$configs/desktop" && stow --delete --target="$HOME" .);;
-  # laptop
-  wpcarro2)
+  $LAPTOP)
     (cd "$configs/laptop" && stow --delete --target="$HOME" .);;
-  # cloudtop
-  wpcarro.c.googlers.com)
+  $CLOUDTOP)
     (cd "$configs/cloudtop" && stow --delete --target="$HOME" .);;
 esac