Respect systemd's RUNTIME_DIRECTORY environment variable

Systemd will export the environment variable RUNTIME_DIRECTORY when
running a deamon. This avoids the need for shipping a tmpfiles.d entry
to create the run directory.

Link: https://www.freedesktop.org/software/systemd/man/latest/systemd.exec.html#RuntimeDirectory=

Author: ekohl

lib/puppet/util/run_mode.rb

@@ -81,7 +81,7 @@ def public_dir
       end
 
       def run_dir
-        which_dir("/var/run/puppetlabs", "~/.puppetlabs/var/run")
+        ENV.fetch('RUNTIME_DIRECTORY') { which_dir("/var/run/puppetlabs", "~/.puppetlabs/var/run") }
       end
 
       def log_dir