Automatic toolchain upgrade to nightly-2024-04-20 (model-checking#3154)

Update Rust toolchain from nightly-2024-04-19 to nightly-2024-04-20
without any other source changes.
This is an automatically generated pull request. If any of the CI checks
fail, manual intervention is required. In such a case, review the
changes at https://github.com/rust-lang/rust from
rust-lang/rust@e3181b0
up to
rust-lang/rust@f9b1614.

rust-toolchain.toml

@@ -2,5 +2,5 @@
 # SPDX-License-Identifier: Apache-2.0 OR MIT
 
 [toolchain]
-channel = "nightly-2024-04-19"
+channel = "nightly-2024-04-20"
 components = ["llvm-tools-preview", "rustc-dev", "rust-src", "rustfmt"]