macOS CI: use shasum from system

sha256sum introduced in macOS 15 but there is shasum so we do not depend
on deprecated port sha2.

Author: MartinPulec

.github/scripts/macOS/prepare.sh

@@ -52,7 +52,6 @@ set -- \
         qt \
         sdl3 \
         sdl3_ttf \
-        sha2 `#for cksum in CI YAML` \
         speexdsp \
         vulkan-headers \
         wolfssl \

.github/workflows/macos.yml

@@ -101,7 +101,7 @@ jobs:
         done
         mv UltraGrid.dmg UltraGrid-$VERSION.dmg
     - name: Compute checksum
-      run: sha2 -256 UltraGrid-$VERSION.dmg || true
+      run: shasum -a 256 UltraGrid-$VERSION.dmg
     - name: Upload Release Asset
       id: upload-release
       if: (github.repository == 'CESNET/UltraGrid' && github.ref == 'refs/heads/master') || startsWith(github.ref, 'refs/tags/')