update workflow

Author: joneugster

.github/workflows/build.yml

@@ -40,7 +40,7 @@ jobs:
 
       - name: delete unused mathlib cache
         continue-on-error: true
-        run: find ./.lake/packages/mathlib/build/lib -type f \( -name "*.olean" -o -name "*.olean.hash" \) \! -neweraa ./tmp_timestamp -delete -print
+        run: find ./.lake/packages/mathlib/.lake/build/lib -type f \( -name "*.olean" -o -name "*.olean.hash" \) \! -neweraa ./tmp_timestamp -delete -print
 
       - name: delete timestamp file
         run: rm ./tmp_timestamp