also release on tags matching 0.0.0-rc.0, drop main branch filter

I tried keeping the main branch filter without extra
complexity/duplication, but that looks messier than its worth.

Thought about yaml anchors, but those can't be shared across documents...
(so the `---` separator wipes them) :(

.github/workflows/release.yml

@@ -4,8 +4,7 @@ on:
   push:
     tags:
       - '[0-9]+.[0-9]+.[0-9]+'
-    branches:
-      - master
+      - '[0-9]+.[0-9]+.[0-9]+-rc.[0-9]+'
 
 permissions:
   contents: write