use a switch instead of an environment variable

the switch is more reliable (the migrate step would fail if the switch
was removed, the step would not fail if the environment variable was
ignored).

upgrade.sh

@@ -54,7 +54,7 @@ function check() {
 
 function upgrade() {
     set -ex
-    su - gitea -c "WORKDIR=$WORKDIR $BINDIR/gitea-$VERSION-linux-amd64 --config /etc/gitea/gitea.ini migrate"
+    su - gitea -c "$BINDIR/gitea-$VERSION-linux-amd64 --config /etc/gitea/gitea.ini --work-path $WORKDIR migrate"
     ln --force $BINDIR/gitea-$VERSION-linux-amd64 $BINDIR/gitea
     set +x
 }