Fixes#206
When I use unzipped sbt.bat it seems to runs ok, but using the msi installer version, this variable seems to expand and causes:
```
\sbt\bin\java9-rt-export.jar was unexpected at this time.
```
Fixessbt/sbt#3804
Inside the if, it seems like bunch of variables were set to blank.
Using delayed expansion seems to fix this. This is confirmed by the newly added test.