build: Mark release as draft when creating GitHub releases

The maintainer can then replace the plaintext body (derived from the
tag) with a markdown version and then publish.

As part of 6c4144e943 creating a draft
release was accidentally dropped.

Signed-off-by: Rob Bradford <rbradford@rivosinc.com>

.github/workflows/release.yaml

@@ -88,6 +88,7 @@ jobs:
         if: github.event_name == 'create' && github.event.ref_type == 'tag'
         uses: softprops/action-gh-release@v1
         with:
+          draft: true
           files: |
             ./${{ matrix.platform.name_ch }}
             ./${{ matrix.platform.name_ch_remote }}