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

On sneak's ruling in #130, check
builds may 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 now come before ARG CACHEBUST, so Docker keeps those layers until
the base image, the install line, go.mod or go.sum changes. The 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
and reruns on every check build, and both go test runs keep -count=1. The
policy, both checklists, the Go style guide, the README and the script
comments state the rule the same way.

Model: opus-5-5

script/lint

@@ -5,11 +5,13 @@
 # make its answer untrustworthy.
 #
 # The phase is not the last stage in the file, so it is built only when
-# --target names it. A new CACHEBUST value reruns every RUN step, since
-# a cached lint layer is a lint 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.
+# --target names it. A new CACHEBUST value reruns every RUN step after
+# the ARG CACHEBUST line, since a cached lint layer is a lint 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 before every step that lints, tests or
+# compiles; a stage without that line is served from cache on an
+# unchanged tree, with no warning.
 # --output type=cacheonly writes no image, since nothing uses one.
 set -eu