chore: clean up coder build directory on shutdown (#21490)

Adds a step to delete the `build/` directory inside the Coder repo on
shutdown.

---------

Co-authored-by: Dean Sheather <dean@deansheather.com>

dogfood/coder/main.tf

@@ -632,6 +632,9 @@ resource "coder_agent" "dev" {
     # accumulating waste and growing too large.
     go clean -cache
 
+    # Clean up the coder build directory as this can get quite large
+    rm -rf "${local.repo_dir}/build"
+
     # Clean up the unused resources to keep storage usage low.
     #
     # WARNING! This will remove: