SL-20546: Append generated release notes body to our explicit body.

For a tag build that generates a release page, try to deduce the git branch to
which the tag we're building corresponds and add that to release notes.

.github/workflows/build.yaml

@@ -24,6 +24,7 @@ jobs:
     outputs:
       viewer_channel: ${{ steps.build.outputs.viewer_channel }}
       viewer_version: ${{ steps.build.outputs.viewer_version }}
+      viewer_branch:  ${{ steps.build.outputs.viewer_branch }}
       imagename: ${{ steps.build.outputs.imagename }}
     env:
       AUTOBUILD_ADDRSIZE: 64
@@ -176,9 +177,17 @@ jobs:
           if [[ "$GITHUB_REF_TYPE" == "tag" && "${GITHUB_REF_NAME:0:12}" == "Second_Life_" ]]
           then viewer_channel="${GITHUB_REF_NAME%#*}"
                export viewer_channel="${viewer_channel//_/ }"
+               # Since GITHUB_REF_NAME is a tag rather than a branch, we need
+               # to discover to what branch this tag corresponds. Get the tip
+               # commit (for the tag) and then ask for branches containing it.
+               # Assume GitHub cloned only this tag and its containing branch.
+               viewer_branch="$(git branch --contains "$(git log -n 1 --format=%h)" |
+                                grep -v '(HEAD')"
           else export viewer_channel="Second Life Test"
+               viewer_branch="${GITHUB_REF_NAME}"
           fi
           echo "viewer_channel=$viewer_channel" >> "$GITHUB_OUTPUT"
+          echo "viewer_branch=$viewer_branch" >> "$GITHUB_OUTPUT"
 
           # On windows we need to point the build to the correct python
           # as neither CMake's FindPython nor our custom Python.cmake module
@@ -365,8 +374,10 @@ jobs:
           body: |
             ${{ needs.build.outputs.viewer_channel }}
             ${{ needs.build.outputs.viewer_version }}
+            ${{ needs.build.outputs.viewer_branch }}
           prerelease: true
           generate_release_notes: true
+          append_body: true
           # the only reason we generate a GH release is to post build products
           fail_on_unmatched_files: true
           files: "assets/*"