Respect GIT_REF env var when building KAS

This MR starts to respect GIT_REF that will be passed from CNG and Omnibus when they build KAS. GIT_REF will enable us to identify which commit the build is based on.

As we are enabling the rolling release of KAS, GITLAB_KAS_VERSION file in canonical GitLab project will be updated to SHA. But, since the VERSION file is not always updated by the release tools (it updates VERSION file only for public release), we can't identify which commit CNG and Omnibus builds are based on. With this MR, it will get the SHA from GIT_REF and it use it as GitRef in Gitlab-agent codebase.

Part of gitlab-com/gl-infra/delivery#20132 (closed).

FYI:

• CNG sets GIT_COMMIT env var when building KAS. gitlab-kas/Dockerfile#L25 We will update CNG to pass GIT_REF.
• Omnibus doesn't set GIT_COMMIT when building KAS. We need to modify Omnibus to set GIT_REF (MR). gitlab-kas.rb#L31-33

Example output:

$ cat VERSION
17.2.0-rc1

$ sh ./build/print_workspace_status.sh
STABLE_BUILD_VERSION v17.2.0-rc1
STABLE_BUILD_GIT_REF f9435609

$ GIT_REF=f943560913664907ed3e3f7c5ee217a928afb861 sh ./build/print_workspace_status.sh
STABLE_BUILD_VERSION v17.2.0-rc1
STABLE_BUILD_GIT_REF f943560913664907ed3e3f7c5ee217a928afb861