stop using XDG_CACHE_HOME as home directory (#299)

* stop using XDG_CACHE_HOME as home directory

XDG_CACHE_HOME is not a substitute for $HOME, see [1].

[1]: https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html

* fix bats testing setup/teardown

since cmdutil.Homedir() would treat $XDG_CACHE_HOME as $HOME, deleting
$XDG_CACHE_HOME would wipe out previous kubens state. now that we're not
doing that, we need to make a real synthetic $HOME and clear it out so
that $HOME/.kube/kubens doesn't persist between runs.

test/common.bash

@@ -2,13 +2,15 @@
 
 # bats setup function
 setup() {
-  export XDG_CACHE_HOME="$(mktemp -d)"
-  export KUBECONFIG="${XDG_CACHE_HOME}/config"
+  TEMP_HOME="$(mktemp -d)"
+  export TEMP_HOME
+  export HOME=$TEMP_HOME
+  export KUBECONFIG="${TEMP_HOME}/config"
 }
 
 # bats teardown function
 teardown() {
-  rm -rf "$XDG_CACHE_HOME"
+  rm -rf "$TEMP_HOME"
 }
 
 use_config() {