Only extract downloaded current tar archive

The current postinst has the bug that all tar archives store in /opt
that begin with ideaIC- are extracted. While this issue will not appear
to most users it may still be a problem to aborted installations which
are only continued after an update.

See also https://gitlab.com/mmk2410/intellij-idea-community/-/issues/5

update-new-version.sh

@@ -26,6 +26,7 @@ main() {
 	"Upstream version $new"
 
     sed -i "s/$old/$new/g" ./debian/preinst
+    sed -i "s/$old/$new/g" ./debian/postinst
 
     debuild -us -uc