Build the builder before replacing the hand-started one, and listen from a file

Two setup faults, each of which looked like the thing being tested failing.

The builder module was assigned without its artifact ever being built, so
nothing could start — and the build has to happen while the hand-started
builder is still alive. Same chicken-and-egg as the registry, resolved the same
way: the builder that exists builds the one that replaces it.

The firewall test's listeners were squeezed through three levels of shell
quoting and never started, so the test failed on its own setup — which reads
exactly like the firewall working.

scenarios/two-nodes.yml

@@ -28,6 +28,9 @@ images:
   # what the mesh's registry is built from — the same chicken-and-egg the bootstrap has, resolved
   # the same way.
   - registry:2
+  # And the builder, because it is a module the mesh assigns rather than a program somebody
+  # starts by hand — which is the only way its credential can be one the mesh delivered.
+  - mesh-builder:development
 
 place:
   all: [host, runtime]