workflows/release: allow building pre-release tags

When we build a pre-release tag, we don't want to upload the packages to
packagecloud, since they would have a number greater than the main
release, but we still want to build them.  Check if the tag is a
pre-release tag and don't upload it to packagecloud.

Author: bk2204

.github/workflows/release.yml

@@ -67,6 +67,7 @@ jobs:
     - run: git clone https://github.com/git-lfs/build-dockers.git "$HOME/build-dockers"
     - run: (cd "$HOME/build-dockers" && ./build_dockers.bsh)
     - run: DOCKER_AUTOPULL=0 ./docker/run_dockers.bsh
-    - run: ./script/packagecloud.rb
+    # If this is a pre-release tag, don't upload anything to packagecloud.
+    - run: '[ -z "${GITHUB_REF%%refs/tags/*-pre*}" ] || ./script/packagecloud.rb'
       env:
         PACKAGECLOUD_TOKEN: ${{secrets.PACKAGECLOUD_TOKEN}}