[1.2.x] chore(e2e): use a fixed digit of SHORT_SHA for image name

[openshift-cherrypick-robot]

This is an automated cherry-pick of #1873

/assign kim-tsao

[openshift-ci]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[openshift-ci]

Hi @openshift-cherrypick-robot. Thanks for your PR.

I'm waiting for a janus-idp member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[kim-tsao]

/ok-to-test

[github-actions]

The image is available at: quay.io/janus-idp/backstage-showcase:pr-1889!