fix(ci-sizer): 🐛 align GARM_URL with template output

Use short service DNS (garm.garm.svc:80) instead of FQDN
(garm.garm.svc.cluster.local:80) to match what the stack template
now generates.

Ref: IPCEICIS-6886

otc/dev.t09.de/stacks/ci-sizer/sizer-receiver/deployment.yaml

@@ -40,7 +40,7 @@ spec:
                   name: sizer-tokens
                   key: hmac-key
             - name: GARM_URL
-              value: "http://garm.garm.svc.cluster.local:80"
+              value: "http://garm.garm.svc:80"
             - name: GARM_USER
               value: "admin"
             - name: GARM_PASSWORD