Correct the build cache wording after review

REPO_POLICIES.md no longer says a Go test phase's image holds every
compiled package; with Go's build cache mounted, those stay in the
mount. The comments in script/lint, script/test, script/cibuild and
script/docker now say that a new CACHEBUST value reruns the RUN steps
only because every stage of the Dockerfile declares ARG CACHEBUST
first, and that a stage without the line is served from cache with no
warning. NEW_REPO_CHECKLIST.md tells a new repository to add the
scripts and every stage's ARG CACHEBUST line in the same commit, in
place of re-vendoring them.

Model: opus-5-5

script/cibuild

@@ -4,6 +4,9 @@
 # the formatter on the host, which a pristine checkout cannot do.
 # A new CACHEBUST value reruns every RUN step, since a cached gate phase
 # is a check that did not run; unlike --no-cache, it keeps Go's build cache.
+# This relies on every stage of the Dockerfile declaring ARG CACHEBUST
+# first; a stage without that line is served from cache on an unchanged
+# tree, with no warning.
 set -eu
 
 SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd -P)"