Point the builder at build/, which is ignored

Pointed at the repository root, the binary it writes is 12 MB of tracked
artifact. Said in the example rather than left to be discovered by a
On branch initialization
Your branch is ahead of 'origin/initialization' by 43 commits.
  (use "git push" to publish your local commits)

Changes to be committed:
  (use "git restore --staged <file>..." to unstage)
	modified:   README.md that looks wrong.

README.md

@@ -142,7 +142,7 @@ is written down here rather than reconstructed a third time.
 export MESH_LAB_HOST_BINARY=<mesh-host>/mesh-host
 export MESH_LAB_BUNDLE=<mesh-host>/examples/substrate-first-node.lock
 export MESH_LAB_MODULES=<mesh-control>/examples/modules
-export MESH_LAB_BUILDER=<mesh-control>/mesh-builder
+export MESH_LAB_BUILDER=<mesh-control>/build/mesh-builder   # build/, which is git-ignored
 
 # Built with `go build -o <path> ./examples/<name>` in mesh-control.
 export MESH_LAB_PROVISIONER=<somewhere>/postgres-provisioner