use variable instead of hardcoded url

update-gluon.sh

@@ -8,7 +8,7 @@ gluon_fork_git_url="ssh://git@gitlab.freifunk-stuttgart.de:22220/firmware/gluon.
 gluon_upstream_git_url="https://github.com/freifunk-gluon/gluon.git"
 site_git_url="ssh://git@gitlab.freifunk-stuttgart.de:22220/firmware/site-ffs.git"
 
-upstream_newest_minor=$(git ls-remote --tags --refs https://github.com/freifunk-gluon/gluon.git 'refs/tags/v2018.2*' | cut -d/ -f3 | sort -r -n -t. -k2 | head -n1)
+upstream_newest_minor=$(git ls-remote --tags --refs "$gluon_upstream_git_url" 'refs/tags/v2018.2*' | cut -d/ -f3 | sort -r -n -t. -k2 | head -n1)
 # branch name in our forked gluon
 fork_newest_minor_branch="$upstream_newest_minor-ffs"
 fork_newest_minor_tag="$upstream_newest_minor-ffs0.1"