Add note about ENV for systemd installs (#91424)

* Add note about ENV for systemd installs

This ES_TMPDIR env variable is not referenced in the /guide/en/elasticsearch/reference/current/setting-system-settings.html#sysconfig section, and when searching for the error mentioned in this page, it might not become too obvious.

* Restructure

---------

Co-authored-by: Abdon Pijpelink <abdon.pijpelink@elastic.co>

docs/reference/setup/sysconfig/executable-jna-tmpdir.asciidoc

@@ -34,6 +34,9 @@ instance:
 export ES_TMPDIR=/usr/share/elasticsearch/tmp
 --------------------------------------------
 
+* For installs done through RPM or DEB packages, the environment variable needs
+to be set through the <<sysconfig,system configuration file>>.
+
 * If you are using `systemd` to run {es} as a service, add the following
 line to the `[Service]` section in a <<systemd,service override file>>:
 +