pin to nightly-2024-06-12 to fix compilation

works around rust-lang/rust#126766 where due to some panic infrastructure changes, some type aliases don't work out
when importing from `core::panic` rather than `std::panic`.  We don't strictly *need* `core::panic`, but it's generally nice to import
from the lowest crate necessary, so we know where things are coming from.

Author: asquared31415

rust-toolchain

@@ -0,0 +1 @@
+nightly-2024-06-12