fix error in uploadtool (#1405)

The error meant releases were always deleted, preventing
a second job from adding an asset to an existing release.
This use case is important now as we are generating snaps
for both amd64 and arm64.

tools/uploadtool/upload_github.sh

@@ -79,7 +79,7 @@ echo "release_url: $release_url"
 target_commit_sha=$(echo "$release_infos" | grep '"target_commitish":' | head -n 1 | cut -d '"' -f 4 | cut -d '{' -f 1)
 echo "target_commit_sha: $target_commit_sha"
 
-if [ "{$GITHUB_SHA}" != "$target_commit_sha" ] ; then
+if [ "${GITHUB_SHA}" != "$target_commit_sha" ] ; then
 
   echo "GITHUB_SHA != target_commit_sha, hence deleting $RELEASE_NAME..."