Fix emscripten parallel build cache locking

Remove $PARALLEL flag from emcc builds to avoid emscripten cache
locking conflicts during SDL2_mixer port compilation.

The emscripten-ports/SDL2_mixer archived repository triggers
parallel compilation of port dependencies, causing cache lock
contentions when multiple emcc processes attempt simultaneous
access. Serial compilation ensures stable builds.

Author: jserv

.github/workflows/main.yml

@@ -87,12 +87,12 @@ jobs:
       if: ${{ always() }}
     - name: default build using emcc
       run: |
-            make CC=emcc $PARALLEL
+            make CC=emcc
       if: ${{ always() }}
     - name: default build for system emulation using emcc
       run: |
             make distclean
-            make CC=emcc ENABLE_SYSTEM=1 $PARALLEL
+            make CC=emcc ENABLE_SYSTEM=1
             make distclean ENABLE_SYSTEM=1
       if: ${{ always() }}
     - name: default build with -g
@@ -401,12 +401,12 @@ jobs:
        if: ${{ always() }}
      - name: default build using emcc
        run: |
-             make CC=emcc $PARALLEL
+             make CC=emcc
        if: ${{ always() }}
      - name: default build for system emulation using emcc
        run: |
              make distclean
-             make CC=emcc ENABLE_SYSTEM=1 $PARALLEL
+             make CC=emcc ENABLE_SYSTEM=1
              make distclean ENABLE_SYSTEM=1
        if: ${{ always() }}
      - name: check + tests