Revert "style: remove unused variable"

This reverts commit e77fdd12de.

src/build.js

@@ -55,6 +55,7 @@ const dl = async (project) => {
   const filename = project + '.zip'
   const link = `https://gitlab.com/malware-filter/${project}/-/jobs/artifacts/main/download?job=pages`
   const pipelineUrl = `https://gitlab.com/malware-filter/${project}/badges/main/pipeline.svg`
+  let isMirror = false
 
   console.log(`Downloading ${filename} from "${link}"`)
   try {
@@ -72,6 +73,7 @@ const dl = async (project) => {
 
     const mirrorLink = `https://nightly.link/curbengh/${project}/workflows/pages/main/public.zip`
     console.log(`Downloading ${filename} from "${mirrorLink}"`)
+    isMirror = true
 
     try {
       await pipeline(