Correct the phase-build comments in script/lint and script/test (closes #141)

The comments said a phase that is not the last stage is built only when
--target names it. The canonical Dockerfile's last stage copies from both
phases, so a plain build builds them too. The comments now say what
--target does in every repository: it builds that phase and the stages
it depends on, nothing else.

Model: opus-5-5

script/test

@@ -1,8 +1,8 @@
 #!/bin/sh
 # script/test: run the test suite. Testing is a phase of the Dockerfile
 # and this builds that phase alone, on the same terms as script/lint:
-# --target because a phase that is not the last stage is built only when
-# named, and a new CACHEBUST value because a cached test layer is a test
+# --target so that only the test phase and the stages it depends on are
+# built, and a new CACHEBUST value because a cached test layer is a test
 # that did not run; unlike --no-cache, it keeps the layers above the ARG
 # CACHEBUST line and the cache mounts. This relies on every stage of the
 # Dockerfile declaring ARG CACHEBUST before every step that lints, tests