fix: handle CACHE STRING syntax in LEAN_VERSION_IS_RELEASE check (#12800)

This PR fixes a false positive in `release_checklist.py` where the check
for the dev cycle being started would fail even when it was correctly
set up.

The script was looking for `set(LEAN_VERSION_IS_RELEASE 0)` as an exact
prefix match, but CMakeLists.txt uses the CMake cache variable form:
`set(LEAN_VERSION_IS_RELEASE 0 CACHE STRING "")`. The fix uses a regex
that handles both syntaxes.

This was discovered during the v4.29.0-rc4 release when the checklist
incorrectly reported that a "begin dev cycle" PR was needed, even though
PR #12526 had already set `LEAN_VERSION_IS_RELEASE 0` and
`LEAN_VERSION_MINOR 30` on master.

🤖 Prepared with Claude Code

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

script/release_checklist.py

@@ -1014,7 +1014,7 @@ def main():
             version_minor_correct = False
             
         is_release_correct = any(
-            l.strip().startswith("set(LEAN_VERSION_IS_RELEASE 0)") 
+            re.match(r'set\(LEAN_VERSION_IS_RELEASE\s+0[\s)]', l.strip())
             for l in cmake_lines
         )