Plugins: Use sysprop like with es.path.home to pass conf dir (#18870)

Currently we always pass -E to the the plugin cli with the conf dir, but
this causes a very confusing error message when not giving a specific
command to the plugin cli. This change makes path.conf pass just like
path.home. These are special settings, so passing via sysprops is the
right thing to do (it is all about how we pass between shell and java
cli).

closes #18689

distribution/src/main/resources/bin/elasticsearch-plugin

@@ -82,9 +82,10 @@ HOSTNAME=`hostname | cut -d. -f1`
 export HOSTNAME
 
 declare -a args=("$@")
+path_props=(-Des.path.home="$ES_HOME")
 
 if [ -e "$CONF_DIR" ]; then
-  args=("${args[@]}" -Edefault.path.conf="$CONF_DIR")
+  path_props=("${path_props[@]}" -Des.path.conf="$CONF_DIR")
 fi
 
-exec "$JAVA" $ES_JAVA_OPTS -Delasticsearch -Des.path.home="$ES_HOME" -cp "$ES_HOME/lib/*" org.elasticsearch.plugins.PluginCli "${args[@]}"
+exec "$JAVA" $ES_JAVA_OPTS -Delasticsearch "${path_props[@]}" -cp "$ES_HOME/lib/*" org.elasticsearch.plugins.PluginCli "${args[@]}"

distribution/src/main/resources/bin/elasticsearch-plugin.bat

@@ -17,9 +17,14 @@ for %%I in ("%SCRIPT_DIR%..") do set ES_HOME=%%~dpfI
 
 TITLE Elasticsearch Plugin Manager ${project.version}
 
+SET path_props=-Des.path.home="%ES_HOME%"
+IF DEFINED CONF_DIR (
+  SET path_props=!path_props! -Des.path.conf="%CONF_DIR%"
+)
+
 SET args=%*
 SET HOSTNAME=%COMPUTERNAME%
 
-"%JAVA%" %ES_JAVA_OPTS% -Des.path.home="%ES_HOME%" -cp "%ES_HOME%/lib/*;" "org.elasticsearch.plugins.PluginCli" !args!
+"%JAVA%" %ES_JAVA_OPTS% !path_props! -cp "%ES_HOME%/lib/*;" "org.elasticsearch.plugins.PluginCli" !args!
 
 ENDLOCAL