Check tag version early in the build
Why is this change being made?
In https://gitlab.com/gitlab-org/security-products/analyzers/container-scanning/-/pipelines/536066933, @sashi_kumar pointed-out that the check_version script was being run too late, and only for one specific job.
As a result, version 5.0.2 was pushed to latest, major, and minor even though it didn't match the gem version.
This MR fixes the problem by running the version check script in its own job, in the first stage of the build, as seen on this fork: https://gitlab.com/thiagoconde/container-scanning/-/pipelines/537019758.
