build: fix linting issues reported by running golangci-lint with different GOOS

backend/cache/cache_unsupported.go

@@ -3,4 +3,5 @@
 
 //go:build plan9 || js
 
+// Package cache implements a virtual provider to cache existing remotes.
 package cache

backend/cache/utils_test.go

@@ -1,3 +1,6 @@
+//go:build !plan9 && !js
+// +build !plan9,!js
+
 package cache
 
 import bolt "go.etcd.io/bbolt"