chore(relay-staging): declare staging C4 on image E so the director can deploy it (#27462)

The staging director deploy binds its digest to staging-gce-c4's checked-in image. E is main
2bdb55350d (#27405 event-loop max, #27406 sidecar), already mirrored to staging. C4 itself
changes only through its own reviewed refresh workflow.

cloud/dev/scripts/relay-staging-c4-refresh-workflow.test.mjs

@@ -42,8 +42,9 @@ const asiaCells = ['production-gce-c27', 'production-gce-c28', 'production-gce-c
 const c30Digest = '4158d8a2e18e9caec439d257f0c1e45d92ffea8c0262f057b2f08c76a134bcf0'
 // C31 launches on the digest the director served when it was declared.
 const c31Digest = 'f30b5cb1ec52b6b6145efecfa1b8be9e3d309403beffd8abcc64197a2087e269'
-// Staging C4 runs the step-5 image D for the staging e2e.
-const stagingC4Digest = 'ac2ab357d3ed9f104ed3d5c440ef3d1ad493d9661e86d1c404d8dfc6b7c59c02'
+// Staging C4 is declared on image E (event-loop max and the director sidecar); the staging
+// director deploy binds its digest to this line.
+const stagingC4Digest = '94bc2b2d5d42e87ce9528372734862b7616810b81a5b03c3ed140c6c5dd82fc8'
 
 function cellBlock(tfvars, cellId) {
   const start = tfvars.indexOf(`"${cellId}"`)

cloud/infra/terraform/environments/staging.tfvars

@@ -85,7 +85,7 @@ relay_gce_cells = {
     boot_image                  = "https://www.googleapis.com/compute/v1/projects/cos-cloud/global/images/cos-stable-121-18867-528-21"
     capacity_requests           = 6000
     database_pool_max           = 10
-    image                       = "us-central1-docker.pkg.dev/onorca-cloud-staging/orca-cloud/relay@sha256:ac2ab357d3ed9f104ed3d5c440ef3d1ad493d9661e86d1c404d8dfc6b7c59c02"
+    image                       = "us-central1-docker.pkg.dev/onorca-cloud-staging/orca-cloud/relay@sha256:94bc2b2d5d42e87ce9528372734862b7616810b81a5b03c3ed140c6c5dd82fc8"
     initially_enabled           = false
     connection_hard_cap         = 3000
     connection_unobserved_bound = 60