fix(sizer): 🐛 use internal K8s service URL for GARM connection

Switch GARM conditional from explicit GARM_URL env var to DOMAIN_GITEA
presence check. When Forgejo is deployed, GARM is always available at
its cluster-internal service (http://garm.garm.svc:80). Hardcode admin
user since GARM always uses that. GitLab-only deploys skip the block.

Ref: IPCEICIS-6886

template/stacks/ci-sizer/sizer-receiver/deployment.yaml

@@ -39,17 +39,17 @@ spec:
                 secretKeyRef:
                   name: sizer-tokens
                   key: hmac-key
-{{{ if index .Env "GARM_URL" }}}
+{{{- if index .Env "DOMAIN_GITEA" }}}
             - name: GARM_URL
-              value: "{{{ .Env.GARM_URL }}}"
+              value: "http://garm.garm.svc:80"
             - name: GARM_USER
-              value: "{{{ .Env.GARM_USER }}}"
+              value: "admin"
             - name: GARM_PASSWORD
               valueFrom:
                 secretKeyRef:
                   name: garm-fixed-credentials
                   key: admin_password
-{{{ end }}}
+            {{{- end }}}
             - name: RECEIVER_OIDC_ISSUER
               value: "https://dex.{{{ .Env.DOMAIN }}}"
             - name: RECEIVER_OIDC_CLIENT_ID