ci: don't require EXTRA_ prefix on input environment variables

.github/workflows/n3ds.yml

@@ -47,13 +47,9 @@ jobs:
           cat /tmp/n3ds_env.txt >> $GITHUB_ENV
       - name: Verify sdl2-config
         run: |
-          export EXTRA_CFLAGS="$CFLAGS"
-          export EXTRA_LDFLAGS="$LDFLAGS"
           export PATH=${{ env.SDL2_DIR }}/bin:$PATH
           cmake/test/test_sdlconfig.sh
       - name: Verify sdl2.pc
         run: |
-          export EXTRA_CFLAGS="$CFLAGS"
-          export EXTRA_LDFLAGS="$LDFLAGS"
           export PKG_CONFIG_PATH=${{ env.SDL2_DIR }}/lib/pkgconfig
           cmake/test/test_pkgconfig.sh