ci: Add script/determine-release-channel (#18476)

- Refactor duplicated inline script from ci.yml to
`script/determine-release-channel`
- Remove references to non-existent '-nightly' release tags

Release Notes:

- N/A

script/bump-zed-patch-version

@@ -9,11 +9,8 @@ case $channel in
   preview)
     tag_suffix="-pre"
     ;;
-  nightly)
-    tag_suffix="-nightly"
-    ;;
   *)
-    echo "this must be run on either of stable|preview|nightly release branches" >&2
+    echo "this must be run on either of stable|preview release branches" >&2
     exit 1
     ;;
 esac