JOSM - save preferences and plugins locally

I am starting JOSM using the following script:

rem Set up settings + plugin location
set APPDATA=\APPDATA
echo %APPDATA%

rem Download latest version
curl http://josm.openstreetmap.de/download/josm-tested.jar -o josm-tested.jar

rem Run 
java.exe -Xmx1024M -jar josm-tested.jar

The update works, the running itself works, but all preferences are saved to C:UsersMyNameAppDataRoaming despite environment variable being set to APPDATA.

I looked into the source code of JOSM and on the first look it seems like asking for the correct environment variable. Why does it not work?

Background: I want to have JOSM in a folder together with everything, however I do not want to install java on a flash drive and run it from there ...

Edit: If anyone is interested this is the result so far (updates josm only if needed, keeps all settings within own folder, starts josm and tracerserver (only relevant for the Czech Rep.)

@echo off

echo:Setting up settings + plugin location
set APPDATA=appdata
echo:set to %APPDATA%

echo:Determining latest tested version
curl http://josm.openstreetmap.de/tested -verbose -o tested.txt
set /p testedVersion=<tested.txt

echo:Determining current version
java.exe -Xmx1024M -jar josm-tested.jar -? | findstr /C:"Revision" > current.txt
set /p temp=<current.txt
set currentVersion=%temp:~10,4%

set /a diffVersion=testedVersion-currentVersion
echo %diffVersion%

if %diffVersion% gtr 0 (
    echo:Update needed
) else (
    echo:JOSM is up to date 
    goto runapps
)

echo:Downloading latest version
rem Download to temporary file, then copy and remove - to prevent file corruption when download fails
curl http://josm.openstreetmap.de/download/josm-tested.jar -o josm-tested-download.jar
move /Y josm-tested-download.jar josm-tested.jar
del josm-tested-download.jar

:runapps
echo Starting tracer server
start TracerServer\bin\Osm.Kn.Trace.Server.exe

echo Starting JOSM 
start java.exe -Xmx1024M -jar josm-tested.jar

:end