[OPENJDK-3683] revert change to GHA image name

This variable is used to find the image descriptor which hasn't
been renamed. This commit should fix the GHA jobs.

Signed-off-by: Jonathan Dowland <[email protected]>

Author: jmtd

.github/workflows/ubi9-openjdk-21.yml

@@ -2,7 +2,7 @@ name: UBI9 OpenJDK 21 S2I Image CI
 on: [push, pull_request]
 env:
   LANG: en_US.UTF-8
-  IMAGE: openjdk-21-jlink-rhel9
+  IMAGE: ubi9-openjdk-21
 jobs:
   call-openjdkci:
     uses: ./.github/workflows/image-workflow-template.yml