Update workflow to build both versions

.github/workflows/core-st.yml

+name: Build FFmpeg.wasm (Single Thread)
+on:
+  push:
+    branches: [ n4.3.1-wasm ]
+  pull_request:
+    branches: [ n4.3.1-wasm ]
+
+env:
+  EM_VERSION: 2.0.8
+  EM_CACHE_FOLDER: 'emsdk-cache'
+
+jobs:
+  linux-build_with_docker:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+        with:
+          submodules: recursive
+      - name: Build FFMpeg
+        shell: bash
+        run: |
+          FFMPEG_ST=yes bash build-with-docker.sh
+      - uses: actions/upload-artifact@master
+        with:
+          name: ffmpeg-core
+          path: wasm/packages/core-st/dist

.github/workflows/main.yml → .github/workflows/core.yml

-name: FFmpeg.wasm
-# Controls when the action will run. Triggers the workflow on push or pull request
-# events but only for the master branch
+name: Build FFmpeg.wasm
 on:
   push:
     branches: [ n4.3.1-wasm ]
@@ -17,17 +15,15 @@ jobs:
     steps:
       - uses: actions/checkout@v2
         with:
-          fetch-depth: 0
           submodules: recursive
       - name: Build FFMpeg
         shell: bash
         run: |
-          docker system prune -f  # fix `Too many levels of symbolic links` error
           bash build-with-docker.sh
       - uses: actions/upload-artifact@master
         with:
           name: ffmpeg-core
-          path: wasm/dist
+          path: wasm/packages/core/dist
   # Disable macos build due to there is no ldconfig command
   # during freetype2 build.
   # macos-build: