chore: bump toolchain to nightly-2026-05-29

Garmelon · Garmelon · commit 57a16fbda599 · 2026-05-29T16:25:04.000+02:00
diff --git a/lean-toolchain b/lean-toolchain
@@ -1 +1 @@
-leanprover/lean4:v4.31.0-rc1
+leanprover/lean4:nightly-2026-05-29
