tests: Use nightly-2024-04-14 in AVR test

```
error[E0658]: use of unstable library feature 'proc_macro_byte_character'
   --> /home/runner/.cargo/registry/src/index.crates.io-6f17d22bba15001f/proc-macro2-1.0.80/src/wrapper.rs:871:21
    |
871 |                     proc_macro::Literal::byte_character(byte)
    |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: see issue #115268 <rust-lang/rust#115268> for more information
    = help: add `#![feature(proc_macro_byte_character)]` to the crate attributes to enable
    = note: this compiler was built on 2024-03-21; consider upgrading it if it is out of date

error[E0658]: use of unstable library feature 'proc_macro_c_str_literals'
   --> /home/runner/.cargo/registry/src/index.crates.io-6f17d22bba15001f/proc-macro2-1.0.80/src/wrapper.rs:898:21
    |
898 |                     proc_macro::Literal::c_string(string)
    |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: see issue #119750 <rust-lang/rust#119750> for more information
    = help: add `#![feature(proc_macro_c_str_literals)]` to the crate attributes to enable
    = note: this compiler was built on 2024-03-21; consider upgrading it if it is out of date
```

Also, the latest nightly cannot be used due to rust-lang/compiler-builtins#577.

```
  = note: /usr/lib/gcc/avr/5.4.0/../../../avr/lib/avr6/libm.a(addsf3.o): In function `__addsf3':
          (.text.avr-libc.fplib+0x2): multiple definition of `__addsf3'
          /home/runner/work/portable-atomic/portable-atomic/target/no-std-test/avr-unknown-gnu-atmega2560/debug/deps/libcompiler_builtins-81c93cbf49042e5b.rlib(compiler_builtins-81c93cbf49042e5b.compiler_builtins.3808c58458fddf11-cgu.07.rcgu.o):/home/runner/.cargo/registry/src/index.crates.io-6f17d22bba15001f/compiler_builtins-0.1.109/src/macros.rs:500: first defined here
```

tests/avr/Cargo.toml

@@ -13,8 +13,6 @@ paste = "1"
 ufmt = "0.2"
 
 [target.avr-unknown-gnu-atmega2560.dependencies]
-# When updating the revision, the reminder to update rust-toolchain.toml to use
-# the same toolchain as https://github.com/Rahix/avr-hal/blob/HEAD/rust-toolchain.toml.
 arduino-hal = { git = "https://github.com/taiki-e/avr-hal.git", branch = "dev", features = ["arduino-mega2560"] }
 
 [workspace]

tests/avr/rust-toolchain.toml

@@ -1,6 +1,6 @@
 [toolchain]
 # channel = "nightly-2022-07-10" # LLVM 14
 # channel = "nightly-2023-08-09" # LLVM 17
-channel = "nightly-2024-03-22" # LLVM 18
+channel = "nightly-2024-04-14" # LLVM 18
 components = ["rust-src"]
 profile = "minimal"