fix: parse LEAN_VERSION_MINOR correctly in release_checklist.py (#12785)

This PR fixes a parsing bug in \`release_checklist.py\` introduced by
https://github.com/leanprover/lean4/pull/12700, which reformatted
\`src/CMakeLists.txt\` to use \`CACHE STRING \"\"\`:

\`\`\`cmake
set(LEAN_VERSION_MINOR 30 CACHE STRING "")
\`\`\`

The old code used \`split()[-1].rstrip(")")\` to extract the version
number, which now yields \`""\` (the empty string argument) instead of
the minor version. Use a regex to extract the digit directly.

🤖 Prepared with Claude Code

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

script/release_checklist.py

@@ -966,7 +966,8 @@ def main():
         # Find the actual minor version in CMakeLists.txt
         for line in cmake_lines:
             if line.strip().startswith("set(LEAN_VERSION_MINOR "):
-                actual_minor = int(line.split()[-1].rstrip(")"))
+                m = re.search(r'set\(LEAN_VERSION_MINOR\s+(\d+)', line)
+                actual_minor = int(m.group(1)) if m else 0
                 version_minor_correct = actual_minor >= next_minor
                 break
         else: