chore(postversion): use const instead of let (#700)

Author: hiTommyhu

scripts/postversion.mjs

@@ -6,7 +6,7 @@ $.verbose = true
 await $`pnpm build`
 await $`pnpm snapshot`
 
-let { version } = JSON.parse(await fs.readFile('./package.json'))
+const { version } = JSON.parse(await fs.readFile('./package.json'))
 
 const playgroundDir = path.resolve(__dirname, '../playground/')
 cd(playgroundDir)