release_process: missing 'cd' call for signature generation

The following is incomplete:

1. Create an armoured detached signature for each created or modified
   upgrade-description file.

        find "${RELEASE_CHECKOUT:?}/wiki/src/upgrade/" \
           -type f -name upgrades.yml | \
           while read udf; do
               if [ -n "$(git status --porcelain "${udf:?}")" ]; then
                   for x in 1 2 3; do
                       gpg -u "${TAILS_SIGNATURE_KEY:?}" --armor \
                           --detach-sign "${udf:?}" \
                       && break
                   done
                   mv --force "${udf:?}.asc" "${udf:?}.pgp"
                   ( \
                     cd ${IUK_CHECKOUT:?} && \
                     ./bin/tails-iuk-check-upgrade-description-file "${udf:?}" \
                   ) || break
               fi
           done

The find call with an absolute path could look good, but there are some git commands being called in the loop, which needs to happen in the checkout. I suggest adding cd ${RELEASE_CHECKOUT:?} at the very beginning to avoid such issues.

Original created by @CyrilBrulebois on 16406 (Redmine)