pyk/klean/template/lean-toolchain: pin Lean to v4.30.0

The template's lean-toolchain was `stable`, so `lake build` in the generated
project floated to the latest stable Lean. v4.31.0 added `ByteArray.decEq` to
core, colliding with the template's own definition and breaking the
klean/test_generate integration test. Pin to v4.30.0, the last stable before
the conflict.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: ehildenb

pyk/src/pyk/klean/template/{{ cookiecutter.package_name }}/lean-toolchain

@@ -1 +1 @@
-stable
+leanprover/lean4:v4.30.0