zsh: skip loading environment if ZDOTDIR is set

Loading the environment on every shell invocation is slow. We can speed
up shell startup a little by skipping this for ever shell because it's
loaded on login by systemd.

Signed-off-by: Joe Groocock <me@frebib.net>

zsh/.zshenv

@@ -1,5 +1,11 @@
-set -o allexport
-. ~/.config/environment.d/*.conf
-set +o allexport
+# Assume environment is already loaded if ZDOTDIR is set.
+# This speeds up starting zsh if it's not required. Attempt
+# to load the environment if it's absent, though, in case
+# ZDOTDIR is overriden and .zshrc is not in ~
+if [ -z "$ZDOTDIR" ]; then
+    set -o allexport
+    . ~/.config/environment.d/*.conf
+    set +o allexport
+fi
 
 # vim: ft=sh