Keep modules, lint cache and system packages in check builds (closes #130)

On sneak's ruling in #130, check
builds keep three more things besides Go's build cache. In each stage of
the canonical Go Dockerfile, the system-package install and the module
download come before ARG CACHEBUST, so Docker keeps those layers until
the base image, the install line, go.mod or go.sum changes. Modules are
kept as a layer, not a cache mount, because Go does not check a cache
mount's files again. golangci-lint run also mounts golangci-lint's cache.
Every step that lints, tests or compiles still comes after ARG CACHEBUST,
both go test runs keep -count=1, and a step that also installs node
packages stays below the line. The policy, checklists, Go style guide,
README and script comments say the same.

Model: opus-5-5

script/cibuild

@@ -2,11 +2,12 @@
 # script/cibuild: run the CI build. It bootstraps first: a CI runner
 # checks out and runs this and nothing else, and script/fmt-check runs
 # 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.
+# A new CACHEBUST value reruns every RUN step after the ARG CACHEBUST
+# line, since a cached gate phase is a check that did not run; unlike
+# --no-cache, it keeps the layers above that line and the cache mounts.
 # 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.
+# before every step that lints, tests or compiles; 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)"