Fix ceres_install script: fetch instead of pull
Fix ceres_install script: fetch instead of pull

`git pull` fails when a tag is checked out, as there is no branch to merge.
Use `fetch` to ensure the script works on subsequent runs, even if the
CERES_VERSION variable is changed to reference a tag that didn't exist before.

wavelab/libwave,wavelab/libwave,wavelab/libwave,wavelab/libwave