chore(dogfood): add project to mux module (#21894)

Adds `add-project` to the `mux` module in the dogfood Coder template so
Mux opens the cloned repo by default.

- Uses `local.repo_dir` (defaults to `/home/coder/coder`) so it stays
correct if the repo base dir parameter changes.

Testing:
- `terraform fmt -check dogfood/coder/main.tf`

dogfood/coder/main.tf

@@ -379,6 +379,7 @@ module "mux" {
   agent_id     = coder_agent.dev.id
   subdomain    = true
   display_name = "Mux"
+  add-project  = local.repo_dir
 }
 
 module "code-server" {