chore: auto compute prelease setting (#2452)

This makes the release workflow automatically compute the prelease
setting based on the
tag name being processed. It looks for the "-rc" text in the tag to do
this.

Co-authored-by: Ignas Anikevicius <240938+aignas@users.noreply.github.com>

.github/workflows/release.yml

diff --git a/.github/workflows/release.yml b/.github/workflows/release.yml
index 96624b3..29b70cc 100644
--- a/.github/workflows/release.yml
+++ b/.github/workflows/release.yml
@@ -42,5 +42,6 @@
           # Use GH feature to populate the changelog automatically
           generate_release_notes: true
           body_path: release_notes.txt
+          prerelease: ${{ contains(github.ref, '-rc') }}
           fail_on_unmatched_files: true
           files: rules_python-*.tar.gz