scripts: fix update-gluon

We were expecting a checked out branch which obviously isn't always the
case. Instread try and find a tag.

scripts/update-gluon.sh

@@ -30,8 +30,8 @@ git -C "$gluon_path" fetch origin --tags
 
 git -C "$gluon_path" fetch upstream "$tag_to_update_to"
 
-current_branch_name=$(git -C "$gluon_path" rev-parse --abbrev-ref HEAD)
-current_upstream_tag_name=${current_branch_name/-ffs/}
+current_tag_name=$(git -C "$gluon_path" describe)
+current_upstream_tag_name=${current_tag_name/-ffs*/}
 
 git -C "$gluon_path" checkout -b "$tag_to_update_to-ffs"