github: containers: Fix job condition.

Unlike 'jobs.<job_id>.steps.if', the 'jobs.<job_id>.if' doesn't
allow use of the 'env' context.  This breaks the workflow parsing:

   Invalid workflow file: .github/workflows/containers.yml#L18
   The workflow is not valid. .github/workflows/containers.yml
   (Line: 18, Col: 9): Unrecognized named-value: 'env'. Located at
   position 28 within expression:
     github.repository_owner == env.IMAGE_NAMESPACE

Use the name directly, as we do in other workflows.

Fixes: 2e5a69d ("github: Don't run scheduled workflows on forks.")
Signed-off-by: Ilya Maximets <[email protected]>

.github/workflows/containers.yml

@@ -15,7 +15,7 @@ env:
 
 jobs:
   container:
-    if: github.repository_owner == env.IMAGE_NAMESPACE
+    if: github.repository_owner == 'ovn-org'
     runs-on: ubuntu-24.04
     strategy:
       matrix: