Attempt to fix duplicate releases issue (#18025)

This hopefully fixes https://github.com/element-hq/synapse/issues/17991,
as we first upgraded to v2 and are now back to 0.1.15.
(This was lost in https://github.com/element-hq/synapse/pull/17923,
related https://github.com/element-hq/synapse/pull/17995)

.github/workflows/release-artifacts.yml

@@ -221,3 +221,7 @@ jobs:
             Sdist/*
             Wheel*/*
             debs.tar.xz
+          # if it's not already published, keep the release as a draft.
+          draft: true
+          # mark it as a prerelease if the tag contains 'rc'.
+          prerelease: ${{ contains(github.ref, 'rc') }}

changelog.d/18025.misc

@@ -0,0 +1 @@
+Fix release process to not create duplicate releases.