Upgrade Rust toolchain to 2025-06-17 (model-checking#4163)

Relevant upstream PR: rust-lang/rust#137944
(Sized Hierarchy: Part I). This PR implements a part of
[RFC 3729](rust-lang/rfcs#3729), which
prescribes a hierarchy of `Sized` traits. Notably, this disallows
instantiation of `size_of_val` and `align_of_val` with extern types.
Consequently, the code in test `unsized_foreign.rs` no longer compiles
instead of panicking, as it previously did. This test is therefore
removed.

Work in progress: figure out whether we can make `foreign_type.rs` work.

Resolves: model-checking#4165

Author: tautschnig

rust-toolchain.toml

@@ -2,5 +2,5 @@
 # SPDX-License-Identifier: Apache-2.0 OR MIT
 
 [toolchain]
-channel = "nightly-2025-06-17"
+channel = "nightly-2025-06-18"
 components = ["llvm-tools", "rustc-dev", "rust-src", "rustfmt"]