fix: envvar file loader processing TZ incorrectly

setenv treats the TZ (and only TZ) envvar especially, so we have to too,
since I'm intentionally avoiding iteratively setenv'ing envvars to avoid
the unnecessary extra work it does.

Fix: #5760

core/core-lib.el

@@ -125,6 +125,7 @@ unreadable. Returns the names of envvars that were changed."
     (with-temp-buffer
       (insert-file-contents file)
       (when-let (env (read (current-buffer)))
+        (let ((tz (getenv-internal "TZ")))
           (setq-default
            process-environment
            (append env (default-value 'process-environment))
@@ -134,6 +135,9 @@ unreadable. Returns the names of envvars that were changed."
            shell-file-name
            (or (getenv "SHELL")
                (default-value 'shell-file-name)))
+          (when-let (newtz (getenv-internal "TZ"))
+            (unless (equal tz newtz)
+              (set-time-zone-rule newtz))))
         env))))
 
 (defun doom-run-hook (hook)