Move kscript cache to XDG_CACHE_HOME

Addresses #17
 - Adds `KSCRIPT_CACHE_DIR` pointing to `XDG_CACHE_HOME` to `.zshenv`
 - Note: out-of-the-box support is still an open issue

.zshenv

@@ -73,6 +73,11 @@ export TRAVIS_CONFIG_PATH=${XDG_CONFIG_HOME}/travis
 #    `~/.gradle/daemon` even with `GRADLE_USER_HOME` pointing elsewhere"
 export GRADLE_USER_HOME=${XDG_DATA_HOME}/gradle
 
+# kscript
+#  - Note: Out-of-the-box XDG support is still an open issue, monitor.
+#  - https://github.com/holgerbrandl/kscript/issues/323
+export KSCRIPT_CACHE_DIR=${XDG_CACHE_HOME}/kscript
+
 # Base16
 export BASE16_FZF_HOME=${XDG_CONFIG_HOME}/base16-fzf
 export BASE16_SHELL_HOME=${XDG_CONFIG_HOME}/base16-shell