Stabilize webpack cache name variant

Use path.basename(sourceBuildDir || outdir) in cache.name to avoid
including environment-specific absolute paths, keeping cache keys
consistent across local and CI builds.

Author: PeterDaveHello

build.mjs

@@ -88,11 +88,12 @@ async function runWebpack(isWithoutKatex, isWithoutTiktoken, minimal, sourceBuil
     sassImpl = mod.default || mod
   }
 
+  const dirKey = path.basename(sourceBuildDir || outdir)
   const variantId = [
     isWithoutKatex ? 'no-katex' : 'with-katex',
     isWithoutTiktoken ? 'no-tiktoken' : 'with-tiktoken',
     minimal ? 'minimal' : 'full',
-    sourceBuildDir || outdir,
+    dirKey,
     isProduction ? 'prod' : 'dev',
   ].join('|')