jenkins-build-makebuildserver: make sure fdroiddata is on master

on the GP jenkins, I got this:

+ git pull
From https://gitlab.com/fdroid/fdroiddata
   1df2d03..621ef4f  master     -> origin/master
You are not currently on a branch. Please specify which
branch you want to merge with. See git-pull(1) for details.

    git pull <remote> <branch>

jenkins-build-makebuildserver

@@ -51,6 +51,7 @@ echo "apt_package_cache = True" >> $WORKSPACE/makebuildserver.config.py
 # this can be handled in the jenkins job, or here:
 if [ -e fdroiddata ]; then
     cd fdroiddata
+    git checkout master
     git pull
     cd ..
 else