If llvm already built through cache don't run the mamba install step

Author: anutosh491

.github/workflows/emscripten.yml

@@ -246,6 +246,7 @@ jobs:
         sudo apt-get clean
 
     - name: install mamba
+      if: ${{ steps.cache.outputs.cache-hit != 'true' }}
       uses: mamba-org/setup-micromamba@main
       with:
         environment-file: environment-wasm-build.yml