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. The headers no longer say each script
builds its phase alone, which is false wherever the phase depends on
another stage.

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
+# and this builds that phase, on the same terms as script/lint:
+# --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