chore(dogfood): add prebuild coder_workspace_tags (#20308)

## Description

Add a `coder_workspace_tags` data block to the Write Coder on Coder
template that conditionally includes the tag `is_prebuild=true` when the
provisioner job is a prebuild (workspace_build) job.

Related internal [PR](https://github.com/coder/dogfood/pull/201) updated
the dogfood deployment with a dedicated pool of provisioner daemons
configured with this tag.
As a result, prebuild-related jobs are now routed exclusively to this
pool, preventing them from competing with user-initiated workspace jobs.

This has been successfully tested internally in dogfood with template
`ssncf-prebuilds-coder`

dogfood/coder/main.tf

@@ -269,6 +269,13 @@ data "coder_workspace_tags" "tags" {
   }
 }
 
+data "coder_workspace_tags" "prebuild" {
+  count = data.coder_workspace_owner.me.name == "prebuilds" ? 1 : 0
+  tags = {
+    "is_prebuild" = "true"
+  }
+}
+
 data "coder_parameter" "ide_choices" {
   type        = "list(string)"
   name        = "Select IDEs"