Fix build-gdk-image script

The regex in scripts/build_gdk_image does not work as expected, so the job build-gdk-image failed because it got the wrong image tag:

#4 (closed) ERROR: registry.gitlab.com/gitlab-org/gitlab-development-kit/asdf-bootstrapped-verify/main:59f9233de457781829a606c11e775b4809677dc3: not found

The right image should have the stable- prefix, e.g. stable-59f9233de457781829a606c11e775b4809677dc3 . This wrong image is cleaned up after 60 days. This blocks all backport MRs from merging.

Exit Criteria

• Fix the script to pick the right image tag