Keep Go's build cache between builds for make test (closes #124)

The Dockerfile runs make test with Go's build cache in a BuildKit cache
mount, so a build compiles only what changed since the last one instead
of the standard library and every dependency from nothing. script/test
passes -count=1, so no test result is taken from the cache.

Model: opus-5-5

script/test

@@ -9,7 +9,9 @@ main() {
     # CGO is required (Makefile exports this too)
     export CGO_ENABLED=1
     go vet ./...
-    go test ./... || go test -v ./...
+    # -count=1: run every test, never take a result from Go's test cache,
+    # which the Dockerfile keeps between builds
+    go test -count=1 ./... || go test -count=1 -v ./...
 }
 
 main "$@"