fix: fix sitemap path resolution

closes #2749

src/node/build/generateSitemap.ts

@@ -17,11 +17,12 @@ export async function generateSitemap(siteConfig: SiteConfig) {
   const getLastmod = async (url: string) => {
     if (!siteConfig.lastUpdated) return undefined
 
-    let path = url.replace(/(^|\/)$/, '$1index')
+    let file = url.replace(/(^|\/)$/, '$1index')
-    path = path.replace(/(\.html)?$/, '.md')
+    file = file.replace(/(\.html)?$/, '.md')
-    path = siteConfig.rewrites.inv[path] || path
+    file = siteConfig.rewrites.inv[file] || file
+    file = path.join(siteConfig.srcDir, file)
 
-    return (await getGitTimestamp(path)) || undefined
+    return (await getGitTimestamp(file)) || undefined
   }
 
   await task('generating sitemap', async () => {