chore(core): Improve environment setup

With my shell setup cleaned up so all environment variables are set in
`~/.zshenv`, and all the slower interactive shell setup in `~/.zshrc`,
we can no remove the `-i` flag from exec-path-from-shell's setup,
leading to a much faster setup time.

Also load additional environment variables, and also set
`temporary-file-directory` to match the `TMPDIR` environment variable.

core/siren-core-env.el

@@ -8,9 +8,24 @@
 ;;; Code:
 
 (use-package exec-path-from-shell
+  :custom
+  (exec-path-from-shell-variables '("PATH"
+                                    "MANPATH"
+                                    "TMPDIR"
+                                    "GOPATH"
+                                    "KUBECONFIG"))
+  (exec-path-from-shell-arguments '("-l"))
+  (exec-path-from-shell-check-startup-files nil)
+  (exec-path-from-shell-debug nil)
+
   :config
   (when (memq window-system '(mac ns x))
     (exec-path-from-shell-initialize)))
 
+;; Set temporary-file-directory to match TMPDIR environment variable
+(let ((tmpdir (getenv "TMPDIR")))
+  (when (and tmpdir (not (string-blank-p tmpdir)))
+    (setq temporary-file-directory tmpdir)))
+
 (provide 'siren-core-env)
 ;;; siren-core-env.el ends here